diff --git a/core_search_facets/src/Plugin/facets/facet_source/CoreNodeSearchFacetSource.php b/core_search_facets/src/Plugin/facets/facet_source/CoreNodeSearchFacetSource.php
index 882dd4c..f328499 100644
--- a/core_search_facets/src/Plugin/facets/facet_source/CoreNodeSearchFacetSource.php
+++ b/core_search_facets/src/Plugin/facets/facet_source/CoreNodeSearchFacetSource.php
@@ -158,9 +158,11 @@ class CoreNodeSearchFacetSource extends FacetSourcePluginBase implements CoreSea
       case 'entity_reference':
         $query_types['string'] = 'core_node_search_string';
         break;
+
       case 'created':
         $query_types['string'] = 'core_node_search_date';
         break;
+
     }
 
     return $query_types;
diff --git a/core_search_facets/src/Plugin/facets/query_type/CoreNodeSearchDate.php b/core_search_facets/src/Plugin/facets/query_type/CoreNodeSearchDate.php
index dac0c54..54950c6 100644
--- a/core_search_facets/src/Plugin/facets/query_type/CoreNodeSearchDate.php
+++ b/core_search_facets/src/Plugin/facets/query_type/CoreNodeSearchDate.php
@@ -11,7 +11,7 @@ use Drupal\facets\QueryType\QueryTypePluginBase;
 use Drupal\facets\Result\Result;
 
 /**
- * A string query type for core.
+ * A date query type for core search.
  *
  * @FacetsQueryType(
  *   id = "core_node_search_date",
@@ -40,14 +40,17 @@ class CoreNodeSearchDate extends QueryTypePluginBase {
       return;
     }
 
+    /** @var \Drupal\facets\Utility\FacetsDate $facets_date */
+    $facets_date = \Drupal::getContainer()->get('facets.utility.date');
+
     // Gets facet query and this facet's query info.
     /** @var \Drupal\core_search_facets\FacetsQuery $facet_query */
     $facet_query = $facet_source->getFacetQueryExtender();
     $query_info = $facet_source->getQueryInfo($this->facet);
-    $tables_joined = array();
+    $tables_joined = [];
 
     // Obtain the start and end dates.
-    if (preg_match(FACETAPI_REGEX_DATE_RANGE, $active_item, $matches)) {
+    if (preg_match($facets_date::FACETS_REGEX_DATE_RANGE, $active_item, $matches)) {
       $start = strtotime($matches[1]);
       $end = strtotime($matches[8]);
     }
@@ -80,7 +83,8 @@ class CoreNodeSearchDate extends QueryTypePluginBase {
    */
   public function build() {
 
-    $build = [];
+    /** @var \Drupal\facets\Utility\FacetsDate $facets_date */
+    $facets_date = \Drupal::getContainer()->get('facets.utility.date');
 
     // Makes sure there was at least one match.
     /*if (!$this->adapter->hasMatches) {
@@ -96,7 +100,6 @@ class CoreNodeSearchDate extends QueryTypePluginBase {
     $facet_query = $facet_source->getFacetQueryExtender();
     $facet_query->addFacetField($query_info);
 
-
     foreach ($query_info['joins'] as $table_alias => $join_info) {
       $facet_query->addFacetJoin($query_info, $table_alias);
     }
@@ -105,7 +108,6 @@ class CoreNodeSearchDate extends QueryTypePluginBase {
       // Executes query, iterates over results.
       $result = $facet_query->execute();
 
-
       foreach ($result as $record) {
         $raw_values[$record->value] = $record->count;
       }
@@ -139,28 +141,29 @@ class CoreNodeSearchDate extends QueryTypePluginBase {
       if (NULL === $parent) {
         if (count($raw_values) > 1) {
           // @TODO for the moment hard-coding the gap.
-          $gap = FACETAPI_DATE_DAY;//facetapi_get_timestamp_gap(min($timestamps), max($timestamps));
-        } else {
-          $gap = FACETAPI_DATE_HOUR;
+          $gap = $facets_date::FACETS_DATE_DAY;
+        }
+        else {
+          $gap = $facets_date::FACETS_DATE_HOUR;
         }
       }
 
       // Converts all timestamps to dates in ISO 8601 format.
       $dates = [];
       foreach ($timestamps as $timestamp) {
-        $dates[$timestamp] = facetapi_isodate($timestamp, $gap);
+        $dates[$timestamp] = $facets_date->isoDate($timestamp, $gap);
       }
 
       // Treat each date as the range start and next date as the range end.
       $range_end = [];
       $previous = NULL;
       foreach (array_unique($dates) as $date) {
-        if (NULL !== $previous) {
-          $range_end[$previous] = facetapi_get_next_date_increment($previous, $gap);
+        if (is_null($previous)) {
+          $range_end[$previous] = $facets_date->getNextDateIncrement($previous, $gap);
         }
         $previous = $date;
       }
-      $range_end[$previous] = facetapi_get_next_date_increment($previous, $gap);
+      $range_end[$previous] = $facets_date->getNextDateIncrement($previous, $gap);
 
       $facet_results = [];
       foreach ($raw_values as $value => $count) {
@@ -189,7 +192,7 @@ class CoreNodeSearchDate extends QueryTypePluginBase {
         }
 
         // Adds parent information if not already set.
-        if (NULL !== $parent && !isset($build[$new_value]['#item_parents'])) {
+        if (is_null($parent) && !isset($build[$new_value]['#item_parents'])) {
           $build[$parent]['#item_children'][$new_value] = &$build[$new_value];
           $build[$new_value]['#item_parents'][$parent] = $parent;
         }*/
diff --git a/facets.date.inc b/facets.date.inc
deleted file mode 100644
index 2d735ed..0000000
--- a/facets.date.inc
+++ /dev/null
@@ -1,361 +0,0 @@
-<?php
-
-/**
- * @file
- * Date handling functions.
- */
-
-/**
- * String that represents a time gap of a year between two dates.
- */
-define('FACETAPI_DATE_YEAR', 'YEAR');
-
-/**
- * String that represents a time gap of a month between two dates.
- */
-define('FACETAPI_DATE_MONTH', 'MONTH');
-
-/**
- * String that represents a time gap of a day between two dates.
- */
-define('FACETAPI_DATE_DAY', 'DAY');
-
-/**
- * String that represents a time gap of an hour between two dates.
- */
-define('FACETAPI_DATE_HOUR', 'HOUR');
-
-/**
- * String that represents a time gap of a minute between two dates.
- */
-define('FACETAPI_DATE_MINUTE', 'MINUTE');
-
-/**
- * String that represents a time gap of a second between two dates.
- */
-define('FACETAPI_DATE_SECOND', 'SECOND');
-
-/**
- * Date string for ISO 8601 date formats.
- */
-define('FACETAPI_DATE_ISO8601', 'Y-m-d\TH:i:s\Z');
-
-/**
- * Regex pattern for range queries.
- */
-define('FACETAPI_REGEX_RANGE', '/^[\[\{](\S+) TO (\S+)[\]\}]$/');
-
-/**
- * Regex pattern for date queries.
- */
-define('FACETAPI_REGEX_DATE', '/^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})Z$/');
-
-/**
- * Regex pattern for date ranges.
- */
-define('FACETAPI_REGEX_DATE_RANGE', '/^\[(' . trim(FACETAPI_REGEX_DATE, '/^$') . ') TO (' . trim(FACETAPI_REGEX_DATE, '/^$') . ')\]$/');
-
-/**
- * Helper function to convert dates from Unix timestamps into ISO 8601 format.
- *
- * @param $timestamp
- *   An integer containing the Unix timestamp being converted.
- * @param $gap
- *   A string containing the gap, see FACETAPI_DATE_* constants for valid
- *   values. Defaults to FACETAPI_DATE_SECOND.
- *
- * @return
- *   A string containing the date in ISO 8601 format.
- */
-function facetapi_isodate($timestamp, $gap = FACETAPI_DATE_SECOND) {
-  switch ($gap) {
-    case FACETAPI_DATE_SECOND:
-      $format = FACETAPI_DATE_ISO8601;
-      break;
-
-    case FACETAPI_DATE_MINUTE:
-      $format = 'Y-m-d\TH:i:00\Z';
-      break;
-
-    case FACETAPI_DATE_HOUR:
-      $format = 'Y-m-d\TH:00:00\Z';
-      break;
-
-    case FACETAPI_DATE_DAY:
-      $format = 'Y-m-d\T00:00:00\Z';
-      break;
-
-    case FACETAPI_DATE_MONTH:
-      $format = 'Y-m-01\T00:00:00\Z';
-      break;
-
-    case FACETAPI_DATE_YEAR:
-      $format = 'Y-01-01\T00:00:00\Z';
-      break;
-
-    default:
-      $format = FACETAPI_DATE_ISO8601;
-      break;
-  }
-  return gmdate($format, $timestamp);
-}
-
-/**
- * Return a date gap one increment smaller than the one passed.
- *
- * @param $gap
- *   A string containing the gap, see FACETAPI_DATE_* constants for valid
- *   values.
- * @param $min_gap
- *   A string containing the the minimum gap that can be returned, defaults to
- *   FACETAPI_DATE_SECOND. This is useful for defining the smallest increment
- *   that can be used in a date drilldown.
- *
- * @return
- *   A string containing the smaller date gap, NULL if there is no smaller gap.
- *   See FACETAPI_DATE_* constants for valid values.
- */
-function facetapi_get_next_date_gap($gap, $min_gap = FACETAPI_DATE_SECOND) {
-  // Array of numbers used to determine whether the next gap is smaller than
-  // the minimum gap allowed in the drilldown.
-  $gap_numbers = array(
-    FACETAPI_DATE_YEAR => 6,
-    FACETAPI_DATE_MONTH => 5,
-    FACETAPI_DATE_DAY => 4,
-    FACETAPI_DATE_HOUR => 3,
-    FACETAPI_DATE_MINUTE => 2,
-    FACETAPI_DATE_SECOND => 1,
-  );
-
-  // Gets gap numbers for both the gap and minimum gap, checks if the next gap
-  // is within the limit set by the $min_gap parameter.
-  $gap_num = isset($gap_numbers[$gap]) ? $gap_numbers[$gap] : 6;
-  $min_num = isset($gap_numbers[$min_gap]) ? $gap_numbers[$min_gap] : 1;
-  return ($gap_num > $min_num) ? array_search($gap_num - 1, $gap_numbers) : $min_gap;
-}
-
-/**
- * Determines the best search gap to use for an arbitrary date range.
- *
- * Generally, we use the maximum gap that fits between the start and end date.
- * If they are more than a year apart, 1 year; if they are more than a month
- * apart, 1 month; etc.
- *
- * This function uses Unix timestamps for its computation and so is not useful
- * for dates outside that range.
- *
- * @param int $start_time
- *   A string containing the start date as an ISO date string.
- * @param int $end_time
- *   A string containing the end date as an ISO date string.
- * @param string|NULL $min_gap
- *   (Optional) The minimum gap that should be returned.
- *
- * @return string
- *   A string containing the gap, see FACETAPI_DATE_* constants for valid
- *   values. Returns FALSE of either of the dates cannot be converted to a
- *   timestamp.
- */
-function facetapi_get_timestamp_gap($start_time, $end_time, $min_gap = NULL) {
-  $time_diff = $end_time - $start_time;
-  switch (TRUE) {
-    // NOTE: 31536000 == 60 * 60 * 24 * 365
-    case ($time_diff >= 31536000):
-      $gap = FACETAPI_DATE_YEAR;
-      break;
-
-    case ($time_diff >= 86400 * gmdate('t', $start_time)):
-      $gap = FACETAPI_DATE_MONTH;
-      break;
-
-    case ($time_diff >= 86400):
-      $gap = FACETAPI_DATE_DAY;
-      break;
-
-    case ($time_diff >= 3600):
-      $gap = FACETAPI_DATE_HOUR;
-      break;
-
-    case ($time_diff >= 60):
-      $gap = FACETAPI_DATE_MINUTE;
-      break;
-
-    default:
-      $gap = FACETAPI_DATE_SECOND;
-      break;
-  }
-
-  // Return the calculated gap if a minimum gap was not passed of the calculated
-  // gap is a larger interval than the minimum gap.
-  if (null === $min_gap || facetapi_gap_compare($gap, $min_gap) >= 0) {
-    return $gap;
-  }
-  else {
-    return $min_gap;
-  }
-}
-
-/**
- * Converts ISO date strings to Unix timestamps, passes values to the
- * facetapi_get_timestamp_gap() function to calculate the gap.
- *
- * @param $start_date
- *   A string containing the start date as an ISO date string.
- * @param $end_date
- *   A string containing the end date as an ISO date string.
- * @param string|NULL $min_gap
- *   (Optional) The minimum gap that should be returned.
- *
- * @return string
- *   A string containing the gap, see FACETAPI_DATE_* constants for valid
- *   values. Returns FALSE of either of the dates cannot be converted to a
- *   timestamp.
- *
- * @see facetapi_get_timestamp_gap()
- */
-function facetapi_get_date_gap($start_date, $end_date, $min_gap = NULL) {
-  $range = array(strtotime($start_date), strtotime($end_date));
-  if (!in_array(FALSE, $range, TRUE)) {
-    return facetapi_get_timestamp_gap($range[0], $range[1], $min_gap);
-  }
-  return FALSE;
-}
-
-/**
- * Returns a formatted date based on the passed timestamp and gap.
- *
- * This function assumes that gaps less than one day will be displayed in a
- * search context in which a larger containing gap including a day is already
- * displayed. So, HOUR, MINUTE, and SECOND gaps only display time information,
- * without date.
- *
- * @param $timestamp
- *   An integer containing the Unix timestamp.
- * @param $gap
- *   A string containing the gap, see FACETAPI_DATE_* constants for valid
- *   values, defaults to YEAR.
- *
- * @return
- *   A gap-appropriate display date used in the facet link.
- */
-function facetapi_format_timestamp($timestamp, $gap = FACETAPI_DATE_YEAR) {
-  switch ($gap) {
-    case FACETAPI_DATE_MONTH:
-      return format_date($timestamp, 'custom', 'F Y', 'UTC');
-
-    case FACETAPI_DATE_DAY:
-      return format_date($timestamp, 'custom', 'F j, Y', 'UTC');
-
-    case FACETAPI_DATE_HOUR:
-      return format_date($timestamp, 'custom', 'g A', 'UTC');
-
-    case FACETAPI_DATE_MINUTE:
-      return format_date($timestamp, 'custom', 'g:i A', 'UTC');
-
-    case FACETAPI_DATE_SECOND:
-      return format_date($timestamp, 'custom', 'g:i:s A', 'UTC');
-
-    default:
-      return format_date($timestamp, 'custom', 'Y', 'UTC');
-  }
-}
-
-/**
- * Returns a formatted date based on the passed ISO date string and gap.
- *
- * @param $date
- *   A string containing the date as an ISO date string.
- * @param $gap
- *   A string containing the gap, see FACETAPI_DATE_* constants for valid
- *   values, defaults to YEAR.
- * @param $callback
- *   The formatting callback, defaults to "facetapi_format_timestamp".
- *
- * @return
- *   A gap-appropriate display date used in the facet link.
- *
- * @see facetapi_format_timestamp()
- */
-function facetapi_format_date($date, $gap = FACETAPI_DATE_YEAR, $callback = 'facetapi_format_timestamp') {
-  $timestamp = strtotime($date);
-  return $callback($timestamp, $gap);
-}
-
-/**
- * Returns the next increment from the given ISO date and gap. This function is
- * useful for getting the upper limit of a date range from the given start
- * date.
- *
- * @param $date
- *   A string containing the date as an ISO date string.
- * @param $gap
- *   A string containing the gap, see FACETAPI_DATE_* constants for valid
- *   values, defaults to YEAR.
- *
- * @return
- *   A string containing the date, FALSE if the passed date could not be parsed.
- */
-function facetapi_get_next_date_increment($date, $gap) {
-  if (preg_match(FACETAPI_REGEX_DATE, $date, $match)) {
-
-    // Increments the timestamp.
-    switch ($gap) {
-      case FACETAPI_DATE_MONTH:
-        $match[2] += 1;
-        break;
-      case FACETAPI_DATE_DAY:
-        $match[3] += 1;
-        break;
-      case FACETAPI_DATE_HOUR:
-        $match[4] += 1;
-        break;
-      case FACETAPI_DATE_MINUTE:
-        $match[5] += 1;
-        break;
-      case FACETAPI_DATE_SECOND:
-        $match[6] += 1;
-        break;
-      default:
-        $match[1] += 1;
-        break;
-    }
-
-    // Gets the next incremenet.
-    return facetapi_isodate(
-      gmmktime($match[4], $match[5], $match[6], $match[2], $match[3], $match[1])
-    );
-  }
-  return FALSE;
-}
-
-/**
- * Compares two timestamp gaps.
- *
- * @param string $gap1
- * @param string $gap2
- *
- * @return int
- *   Returns -1 if gap1 is less than gap2, 1 if gap1 is greater than gap2, and 0
- *   if they are equal.
- */
-function facetapi_gap_compare($gap1, $gap2) {
-
-  $gap_numbers = array(
-    FACETAPI_DATE_YEAR => 6,
-    FACETAPI_DATE_MONTH => 5,
-    FACETAPI_DATE_DAY => 4,
-    FACETAPI_DATE_HOUR => 3,
-    FACETAPI_DATE_MINUTE => 2,
-    FACETAPI_DATE_SECOND => 1,
-  );
-
-  $gap1_num = isset($gap_numbers[$gap1]) ? $gap_numbers[$gap1] : 6;
-  $gap2_num = isset($gap_numbers[$gap2]) ? $gap_numbers[$gap2] : 6;
-
-  if ($gap1_num == $gap2_num) {
-    return 0;
-  }
-  else {
-    return ($gap1_num < $gap2_num) ? -1 : 1;
-  }
-}
\ No newline at end of file
diff --git a/facets.services.yml b/facets.services.yml
index 1be585e..f9693d9 100644
--- a/facets.services.yml
+++ b/facets.services.yml
@@ -22,3 +22,5 @@ services:
       - '@plugin.manager.facets.facet_source'
       - '@plugin.manager.facets.processor'
       - '@entity_type.manager'
+  facets.utility.date:
+    class: Drupal\facets\Utility\FacetsDate
diff --git a/src/Utility/FacetsDate.php b/src/Utility/FacetsDate.php
new file mode 100644
index 0000000..37b53a2
--- /dev/null
+++ b/src/Utility/FacetsDate.php
@@ -0,0 +1,380 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\facets\Utility\FacetsDate.
+ */
+
+namespace Drupal\facets\Utility;
+
+/**
+ * Helpers for dates.
+ */
+class FacetsDate {
+
+  /**
+   * String that represents a time gap of a year between two dates.
+   */
+  const FACETS_DATE_YEAR = 'YEAR';
+
+  /**
+   * String that represents a time gap of a month between two dates.
+   */
+  const FACETS_DATE_MONTH = 'MONTH';
+
+  /**
+   * String that represents a time gap of a day between two dates.
+   */
+  const FACETS_DATE_DAY = 'DAY';
+
+  /**
+   * String that represents a time gap of an hour between two dates.
+   */
+  const FACETS_DATE_HOUR = 'HOUR';
+
+  /**
+   * String that represents a time gap of a minute between two dates.
+   */
+  const FACETS_DATE_MINUTE = 'MINUTE';
+
+  /**
+   * String that represents a time gap of a second between two dates.
+   */
+  const FACETS_DATE_SECOND = 'SECOND';
+
+  /**
+   * Date string for ISO 8601 date formats.
+   */
+  const FACETS_DATE_ISO8601 = 'Y-m-d\TH:i:s\Z';
+
+  /**
+   * Regex pattern for range queries.
+   */
+  const FACETS_REGEX_RANGE = '/^[\[\{](\S+) TO (\S+)[\]\}]$/';
+
+  /**
+   * Regex pattern for date queries.
+   */
+  const FACETS_REGEX_DATE = '/^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})Z$/';
+
+  /**
+   * Regex pattern for date ranges.
+   */
+  const FACETS_REGEX_DATE_RANGE = '/^\[(/^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})Z$/) TO (/^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})Z$/)\]$/';
+
+  /**
+   * Converts dates from unix timestamps to ISO 8601 format.
+   *
+   * @param int $timestamp
+   *   An integer containing the Unix timestamp being converted.
+   * @param string $gap
+   *   A string containing the gap, see FACETS_DATE_* constants for valid
+   *   values. Defaults to self::FACETS_DATE_SECOND.
+   *
+   * @return string
+   *   A string containing the date in ISO 8601 format.
+   */
+  public function isoDate($timestamp, $gap = self::FACETS_DATE_SECOND) {
+    switch ($gap) {
+      case static::FACETS_DATE_SECOND:
+        $format = static::FACETS_DATE_ISO8601;
+        break;
+
+      case static::FACETS_DATE_MINUTE:
+        $format = 'Y-m-d\TH:i:00\Z';
+        break;
+
+      case static::FACETS_DATE_HOUR:
+        $format = 'Y-m-d\TH:00:00\Z';
+        break;
+
+      case static::FACETS_DATE_DAY:
+        $format = 'Y-m-d\T00:00:00\Z';
+        break;
+
+      case static::FACETS_DATE_MONTH:
+        $format = 'Y-m-01\T00:00:00\Z';
+        break;
+
+      case static::FACETS_DATE_YEAR:
+        $format = 'Y-01-01\T00:00:00\Z';
+        break;
+
+      default:
+        $format = static::FACETS_DATE_ISO8601;
+        break;
+    }
+    return gmdate($format, $timestamp);
+  }
+
+  /**
+   * Returns a date gap one increment smaller than the one passed.
+   *
+   * @param string $gap
+   *   A string containing the gap, see FACETAPI_DATE_* constants for valid
+   *   values.
+   * @param string $min_gap
+   *   A string containing the the minimum gap that can be returned, defaults to
+   *   FACETAPI_DATE_SECOND. This is useful for defining the smallest increment
+   *   that can be used in a date drilldown.
+   *
+   * @return string
+   *   A string containing the smaller date gap, NULL if there is no smaller gap.
+   *   See FACETAPI_DATE_* constants for valid values.
+   */
+  public function getNextDateGap($gap, $min_gap = self::FACETS_DATE_SECOND) {
+    // Array of numbers used to determine whether the next gap is smaller than
+    // the minimum gap allowed in the drilldown.
+    $gap_numbers = [
+      static::FACETS_DATE_YEAR => 6,
+      static::FACETS_DATE_MONTH => 5,
+      static::FACETS_DATE_DAY => 4,
+      static::FACETS_DATE_HOUR => 3,
+      static::FACETS_DATE_MINUTE => 2,
+      static::FACETS_DATE_SECOND => 1,
+    ];
+
+    // Gets gap numbers for both the gap and minimum gap, checks if the next gap
+    // is within the limit set by the $min_gap parameter.
+    $gap_num = isset($gap_numbers[$gap]) ? $gap_numbers[$gap] : 6;
+    $min_num = isset($gap_numbers[$min_gap]) ? $gap_numbers[$min_gap] : 1;
+
+    return ($gap_num > $min_num) ? array_search($gap_num - 1, $gap_numbers) : $min_gap;
+  }
+
+  /**
+   * Determines the best search gap to use for an arbitrary date range.
+   *
+   * Generally, we use the maximum gap that fits between the start and end date.
+   * If they are more than a year apart, 1 year; if they are more than a month
+   * apart, 1 month; etc.
+   *
+   * This function uses Unix timestamps for its computation and so is not useful
+   * for dates outside that range.
+   *
+   * @param int $start_time
+   *   A string containing the start date as an ISO date string.
+   * @param int $end_time
+   *   A string containing the end date as an ISO date string.
+   * @param string|NULL $min_gap
+   *   (Optional) The minimum gap that should be returned.
+   *
+   * @return string
+   *   A string containing the gap, see FACETAPI_DATE_* constants for valid
+   *   values. Returns FALSE of either of the dates cannot be converted to a
+   *   timestamp.
+   */
+  public function getTimestampGap($start_time, $end_time, $min_gap = NULL) {
+    $time_diff = $end_time - $start_time;
+
+    switch (TRUE) {
+      // NOTE: 31536000 == 60 * 60 * 24 * 365
+      case ($time_diff >= 31536000):
+        $gap = static::FACETS_DATE_YEAR;
+        break;
+
+      case ($time_diff >= 86400 * gmdate('t', $start_time)):
+        $gap = static::FACETS_DATE_MONTH;
+        break;
+
+      case ($time_diff >= 86400):
+        $gap = static::FACETS_DATE_DAY;
+        break;
+
+      case ($time_diff >= 3600):
+        $gap = static::FACETS_DATE_HOUR;
+        break;
+
+      case ($time_diff >= 60):
+        $gap = static::FACETS_DATE_MINUTE;
+        break;
+
+      default:
+        $gap = static::FACETS_DATE_SECOND;
+        break;
+    }
+
+    // Return the calculated gap if a minimum gap was not passed of the calculated
+    // gap is a larger interval than the minimum gap.
+    if (is_null($min_gap) || $this->dateGapCompare($gap, $min_gap) >= 0) {
+      return $gap;
+    }
+    else {
+      return $min_gap;
+    }
+  }
+
+  /**
+   * Converts ISO date strings to Unix timestamps.
+   *
+   * The ::facetapi_get_timestamp_gap() function is used to calculate the gap.
+   *
+   * @param string $start_date
+   *   A string containing the start date as an ISO date string.
+   * @param string $end_date
+   *   A string containing the end date as an ISO date string.
+   * @param string|NULL $min_gap
+   *   (Optional) The minimum gap that should be returned.
+   *
+   * @return string|bool
+   *   A string containing the gap, see FACETAPI_DATE_* constants for valid
+   *   values. Returns FALSE of either of the dates cannot be converted to a
+   *   timestamp.
+   *
+   * @see getTimestampGap()
+   */
+  public function getDateGap($start_date, $end_date, $min_gap = NULL) {
+    $range = [strtotime($start_date), strtotime($end_date)];
+
+    if (!in_array(FALSE, $range, TRUE)) {
+      return $this->getTimestampGap($range[0], $range[1], $min_gap);
+    }
+    return FALSE;
+  }
+
+  /**
+   * Returns a formatted date based on the passed timestamp and gap.
+   *
+   * This function assumes that gaps less than one day will be displayed in a
+   * search context in which a larger containing gap including a day is already
+   * displayed. So, HOUR, MINUTE, and SECOND gaps only display time information,
+   * without date.
+   *
+   * @param int $timestamp
+   *   An integer containing the Unix timestamp.
+   * @param string $gap
+   *   A string containing the gap, see FACETAPI_DATE_* constants for valid
+   *   values, defaults to YEAR.
+   *
+   * @return
+   *   A gap-appropriate display date used in the facet link.
+   */
+  public function formatTimestamp($timestamp, $gap = self::FACETS_DATE_YEAR) {
+    switch ($gap) {
+      case static::FACETS_DATE_MONTH:
+        return format_date($timestamp, 'custom', 'F Y', 'UTC');
+
+      case static::FACETS_DATE_DAY:
+        return format_date($timestamp, 'custom', 'F j, Y', 'UTC');
+
+      case static::FACETS_DATE_HOUR:
+        return format_date($timestamp, 'custom', 'g A', 'UTC');
+
+      case static::FACETS_DATE_MINUTE:
+        return format_date($timestamp, 'custom', 'g:i A', 'UTC');
+
+      case static::FACETS_DATE_SECOND:
+        return format_date($timestamp, 'custom', 'g:i:s A', 'UTC');
+
+      default:
+        return format_date($timestamp, 'custom', 'Y', 'UTC');
+    }
+  }
+
+  /**
+   * Returns a formatted date based on the passed ISO date string and gap.
+   *
+   * @param string $date
+   *   A string containing the date as an ISO date string.
+   * @param string $gap
+   *   A string containing the gap, see FACETAPI_DATE_* constants for valid
+   *   values, defaults to YEAR.
+   * @param string $callback
+   *   The formatting callback, defaults to "facets_format_timestamp".
+   *
+   * @return
+   *   A gap-appropriate display date used in the facet link.
+   *
+   * @see facetsFormatTimestamp()
+   */
+  public function facetsFormatDate($date, $gap = self::FACETS_DATE_YEAR, $callback = 'facets_format_timestamp') {
+    $timestamp = strtotime($date);
+    return $callback($timestamp, $gap);
+  }
+
+  /**
+   * Returns the next increment from the given ISO date and gap. This function is
+   * useful for getting the upper limit of a date range from the given start
+   * date.
+   *
+   * @param string $date
+   *   A string containing the date as an ISO date string.
+   * @param string $gap
+   *   A string containing the gap, see FACETAPI_DATE_* constants for valid
+   *   values, defaults to YEAR.
+   *
+   * @return
+   *   A string containing the date, FALSE if the passed date could not be parsed.
+   */
+  public function getNextDateIncrement($date, $gap) {
+    if (preg_match(static::FACETS_REGEX_DATE, $date, $match)) {
+
+      // Increments the timestamp.
+      switch ($gap) {
+        case static::FACETS_DATE_MONTH:
+          $match[2] += 1;
+          break;
+        case static::FACETS_DATE_DAY:
+          $match[3] += 1;
+          break;
+        case static::FACETS_DATE_HOUR:
+          $match[4] += 1;
+          break;
+        case static::FACETS_DATE_MINUTE:
+          $match[5] += 1;
+          break;
+        case static::FACETS_DATE_SECOND:
+          $match[6] += 1;
+          break;
+        default:
+          $match[1] += 1;
+          break;
+      }
+
+      // Gets the next incremenet.
+      $time = gmmktime(
+        $match[4],
+        $match[5],
+        $match[6],
+        $match[2],
+        $match[3],
+        $match[1]
+      );
+      return $this->isoDate($time);
+    }
+    return FALSE;
+  }
+
+  /**
+   * Compares two timestamp gaps.
+   *
+   * @param string $gap1
+   * @param string $gap2
+   *
+   * @return int
+   *   Returns -1 if gap1 is less than gap2, 1 if gap1 is greater than gap2, and 0
+   *   if they are equal.
+   */
+  public function dateGapCompare($gap1, $gap2) {
+
+    $gap_numbers = [
+      static::FACETS_DATE_YEAR => 6,
+      static::FACETS_DATE_MONTH => 5,
+      static::FACETS_DATE_DAY => 4,
+      static::FACETS_DATE_HOUR => 3,
+      static::FACETS_DATE_MINUTE => 2,
+      static::FACETS_DATE_SECOND => 1,
+    ];
+
+    $gap1_num = isset($gap_numbers[$gap1]) ? $gap_numbers[$gap1] : 6;
+    $gap2_num = isset($gap_numbers[$gap2]) ? $gap_numbers[$gap2] : 6;
+
+    if ($gap1_num == $gap2_num) {
+      return 0;
+    }
+    else {
+      return ($gap1_num < $gap2_num) ? -1 : 1;
+    }
+  }
+
+}
diff --git a/tests/src/Unit/Utility/FacetsDateTest.php b/tests/src/Unit/Utility/FacetsDateTest.php
new file mode 100644
index 0000000..206f86d
--- /dev/null
+++ b/tests/src/Unit/Utility/FacetsDateTest.php
@@ -0,0 +1,105 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\facets\Unit\Utility\FacetsDateTest.
+ */
+
+namespace Drupal\Tests\facets\Unit\Utility;
+
+use Drupal\facets\Utility\FacetsDate;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Unit test for the checkbox widget form.
+ *
+ * @group facets
+ */
+class FacetsDateTest extends UnitTestCase {
+
+  /**
+   * Tests the isoDate method.
+   *
+   * @dataProvider provideIsoDates
+   */
+  public function testIsoDate($iso_date, $gap) {
+    $fd = new FacetsDate();
+    $this->assertEquals($iso_date, $fd->isoDate(564957784, $gap));
+  }
+
+  /**
+   * Tests for ::getNextDateGap.
+   */
+  public function testGetNextDateGap() {
+    $fd = new FacetsDate();
+
+    $gap = $fd->getNextDateGap($fd::FACETS_DATE_SECOND);
+    $this->assertEquals($fd::FACETS_DATE_SECOND, $gap);
+
+    $gap = $fd->getNextDateGap($fd::FACETS_DATE_MINUTE);
+    $this->assertEquals($fd::FACETS_DATE_SECOND, $gap);
+
+    $gap = $fd->getNextDateGap($fd::FACETS_DATE_SECOND, $fd::FACETS_DATE_MINUTE);
+    $this->assertEquals($fd::FACETS_DATE_MINUTE, $gap);
+
+    $gap = $fd->getNextDateGap($fd::FACETS_DATE_MINUTE, $fd::FACETS_DATE_MINUTE);
+    $this->assertEquals($fd::FACETS_DATE_MINUTE, $gap);
+
+    $gap = $fd->getNextDateGap($fd::FACETS_DATE_SECOND, $fd::FACETS_DATE_HOUR);
+    $this->assertEquals($fd::FACETS_DATE_HOUR, $gap);
+
+    $gap = $fd->getNextDateGap($fd::FACETS_DATE_MINUTE, $fd::FACETS_DATE_HOUR);
+    $this->assertEquals($fd::FACETS_DATE_HOUR, $gap);
+
+    $gap = $fd->getNextDateGap($fd::FACETS_DATE_HOUR, $fd::FACETS_DATE_HOUR);
+    $this->assertEquals($fd::FACETS_DATE_HOUR, $gap);
+  }
+
+  /**
+   * Tests for ::getTimestampGap
+   */
+  public function testGetTimestampGap() {
+    $fd = new FacetsDate();
+    $gap = $fd->getTimestampGap(564957784, 651962584);
+    $this->assertEquals($fd::FACETS_DATE_YEAR, $gap);
+
+    $gap = $fd->getTimestampGap(564957784, 651962584, $fd::FACETS_DATE_SECOND);
+    $this->assertEquals($fd::FACETS_DATE_YEAR, $gap);
+
+    $gap = $fd->getTimestampGap(564957784, 564967784);
+    $this->assertEquals($fd::FACETS_DATE_HOUR, $gap);
+
+    $gap = $fd->getTimestampGap(564957784, 564967784, $fd::FACETS_DATE_SECOND);
+    $this->assertEquals($fd::FACETS_DATE_HOUR, $gap);
+
+    $gap = $fd->getTimestampGap(564957784, 564967784, $fd::FACETS_DATE_MONTH);
+    $this->assertEquals($fd::FACETS_DATE_MONTH, $gap);
+  }
+
+  /**
+   * Tests for ::getDateGap.
+   */
+  public function testGetDateGap() {
+    $fd = new FacetsDate();
+    $gap = $fd->getDateGap('1987-11-26T20:43:04Z', '1990-08-29T20:43:04Z');
+    $this->assertEquals($fd::FACETS_DATE_YEAR, $gap);
+  }
+
+  /**
+   * Returns a data provider for the ::testIsoDate().
+   *
+   * @return array
+   */
+  public function provideIsoDates() {
+    return [
+      ['1987-11-26T20:43:04Z', FacetsDate::FACETS_DATE_SECOND],
+      ['1987-11-26T20:43:00Z', FacetsDate::FACETS_DATE_MINUTE],
+      ['1987-11-26T20:00:00Z', FacetsDate::FACETS_DATE_HOUR],
+      ['1987-11-26T00:00:00Z', FacetsDate::FACETS_DATE_DAY],
+      ['1987-11-01T00:00:00Z', FacetsDate::FACETS_DATE_MONTH],
+      ['1987-01-01T00:00:00Z', FacetsDate::FACETS_DATE_YEAR],
+      ['1987-11-26T20:43:04Z', FacetsDate::FACETS_DATE_ISO8601],
+    ];
+  }
+
+}