Computed Field calculates upcoming repeated event date

Tidied the issue title.

Here is the final code I came up with for displaying a Computed Field showing the next upcoming event date in the node display, as shown in the example at the top. I'll try to get around to writing this up for the Computed Field Code Snippets page. Note that this code has been tested against Drupal 6.8 and Date 6.x-2.0-rc6. Your mileage may vary.

COMPUTED CODE:

/** 
 * Returns a datetime string for a repeating event's next upcoming date, based on its rrule.
 * You MUST customize the $datetime_repeating value below to match your field's name.
 */
// IMPORTANT: Note that my repeating event nodes have a datetime field named 'field_datetime_repeating'
// but you should change the line below to your repeating event node's field name for this value. 
    $datetime_repeating = "field_datetime_repeating";

// The following line is needed for date_ical_parse_rrule() below.
require_once('./'. drupal_get_path('module', 'date_api') .'/date_api_ical.inc');
// ** Get the default timezone for the user if set, else the site's **
$timezone = date_default_timezone_name();
// ** Get "RRULE" **
// The following line retrieves a string value with "RRULE:" as prefix.
// e.g. "RRULE:FREQ=MONTHLY;INTERVAL=1;BYDAY=4TU;UNTIL=20111025T040000Z;WKST=SU"
$rrule = $node->{$datetime_repeating}[0]['rrule'];
// ** Set start date **
$start = date_now($timezone);
// ** Set end date and exceptions **
// End date should be the "UNTIL" value from the rrule for this event.
$parts = date_repeat_split_rrule($rrule);
$rrule_array = $parts[0];
$exceptions = $parts[1];
// If there's no UNTIL found, set it to an arbitrary date of one year from now.
if ($rrule_array['UNTIL'] == "") {
  $end = date_format_date(date_make_date(time() + 31622400, $timezone, DATE_UNIX), 'custom', 'Y-m-d H:i:s');
}
else {
  $end = date_format_date(date_ical_date($rrule_array['UNTIL']), 'custom', 'Y-m-d H:i:s');
}

// ** Finally, get the upcoming dates array and take the first upcoming date if any. **
$upcoming = date_repeat_calc($rrule, $start, $end, $exceptions, $timezone);
if (count($upcoming) < 2) {
//  No upcoming dates.
  $node_field[0]['value'] = "";
}
else {
// Retrieve the event's fromdate in the database's UTC timezone, adjust it to the local timezone,
// and prepare a version in datetime format
  $fromdate_text = $node->field_datetime_repeating[0]['value'];
  $fromdate = date_make_date($fromdate_text, 'UTC');
  date_timezone_set($fromdate, timezone_open($timezone));
  $fromdate_text = date_format_date($fromdate, 'custom', 'Y-m-d H:i:s');
// Set the upcoming event's time to the actual event's time, and pass on a datetime STR.
  $fromdate_array = explode(" ", $fromdate_text);
// Note that the first item in the resulting array is always the $start (as per _date_repeat_calc()
// in date/date_repeat/date_repeat_calc.inc), so take the second one.
  $nextdate_array = explode(" ", $upcoming[1]);
  $nextdatetime = $nextdate_array[0] . " " . $fromdate_array[1];
  $node_field[0]['value'] = $nextdatetime;
}

DISPLAY FORMAT:

if ($node_field_item['value'] == "") {
  $display = "No upcoming repeats";
}
else {
//  $display = date_format_date(date_make_date($node_field_item['value'], date_default_timezone_name()), 'small');
// customized format example below
  $display = date_format_date(date_make_date($node_field_item['value'], date_default_timezone_name()), 'custom', 'M j Y g:ia');
}

OTHER RELEVANT COMPUTED FIELD SETTINGS:

Number of values: 1
Display this field: checked
Store using the database settings below: UNCHECKED (important!)

Ignore the note about "Anyway, there are still problems associated with the fact that the "next upcoming date" doesn't update..." in the top item. That was solved elsewhere (more fully described here).

There seem to be some problems with the Computed Field code I gave earlier. A minor one is that it doesn't take into account the event's time properly, so on the day of the event but before the time of the event it reports the Next Event to be the one after that day, as if that day's event is already past. I know how to fix that (I haven't set the actual event time before the calculation, so it is probably using midnight).

The more confusing problem is with repeating events that are annual. When the event is set to repeat every 1 year, the Computed Field code is returning a day several days after the correct one. The Date module's "Repeats" tab is showing the correct repeating days, so it must be something in my code. I'll post a fix if (and when) I find one. Any ideas or suggestions would be welcome.

That sounds terrific! A much better solution than mine, for sure.