.../Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php   | 4 +---
 .../Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php  | 4 ++--
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
index 366bcd9..ef0010e 100644
--- a/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
+++ b/core/modules/datetime/tests/src/Functional/EntityResource/EntityTest/EntityTestDatetimeTest.php
@@ -103,9 +103,7 @@ protected function getNormalizedPostEntity() {
     return parent::getNormalizedPostEntity() + [
       static::$fieldName => [
         [
-          // Omitting the timezone is allowed, this should result in the site's
-          // timezone being used automatically.
-          'value' => static::$dateString,
+          'value' => static::$dateString . '+00:00',
         ],
       ],
     ];
diff --git a/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php b/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
index 0d2a08d..926b7e8 100644
--- a/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
+++ b/core/modules/datetime_range/tests/src/Functional/EntityResource/EntityTest/EntityTestDateRangeTest.php
@@ -103,8 +103,8 @@ protected function getNormalizedPostEntity() {
     return parent::getNormalizedPostEntity() + [
       static::$fieldName => [
         [
-          'value' => '2017-03-01T20:02:00',
-          'end_value' => '2017-03-01T20:02:00',
+          'value' => '2017-03-01T20:02:00+00:00',
+          'end_value' => '2017-03-01T20:02:00+00:00',
         ],
       ],
     ];