[5.x] Allow different timezone from app timezone for displaying date+time in CP and Antlers

config/system.php

@@ -74,6 +74,25 @@
 
     'date_format' => 'F jS, Y',
 
+    /*
+    |--------------------------------------------------------------------------
+    | Client Timezone
+    |--------------------------------------------------------------------------
+    |
+    | If specified, this timezone is used as the timezone for visible date and
+    | time values in the control panel and in Antlers. That allows you to keep
+    | the app timezone in UTC while having, for example, the `Europe/Berlin`
+    | timezone for the control panel.
+    |
+    | That means, if you publish an entry at 2024-05-22 10:15 with the
+    | client timezone being set to `Europe/Berlin` and the app timezone being
+    | set to `UTC`, the entry date will be 2024-05-22 08:15 internally, but
+    | displayed as 2024-05-22 10:15 in the control panel and in antlers.
+    |
+    */
+
+    'client_timezone' => null,
+
     /*
     |--------------------------------------------------------------------------
     | Default Character Set

src/Data/AbstractAugmented.php

@@ -2,6 +2,7 @@
 
 namespace Statamic\Data;
 
+use Carbon\Carbon;
 use Statamic\Contracts\Data\Augmented;
 use Statamic\Fields\Value;
 use Statamic\Statamic;
@@ -55,6 +56,14 @@ private function transientValue($key)
             $value->setFieldtype($deferred->fieldtype());
             $value->setAugmentable($deferred->augmentable());
 
+            if ($deferred->raw() instanceof Carbon) {
+                $clientTimezone = config('statamic.system.client_timezone');
+                $appTimezone = config('app.timezone');
+                if ($clientTimezone && $clientTimezone !== $appTimezone) {
+                    $deferred->raw()->setTimezone($clientTimezone);
+                }
+            }
+
             return $deferred->raw();
         };
 

src/Fieldtypes/Date.php

@@ -145,7 +145,7 @@ private function preProcessSingle($value)
             $value = $value['start'];
         }
 
-        $date = $this->parseSaved($value);
+        $date = $this->parseSaved($value, true);
 
         return $this->splitDateTimeForPreProcessSingle($date);
     }
@@ -168,8 +168,8 @@ private function preProcessRange($value)
         }
 
         return $this->splitDateTimeForPreProcessRange([
-            'start' => $this->parseSaved($value['start'])->format($vueFormat),
-            'end' => $this->parseSaved($value['end'])->format($vueFormat),
+            'start' => $this->parseSaved($value['start'], true)->format($vueFormat),
+            'end' => $this->parseSaved($value['end'], true)->format($vueFormat),
         ]);
     }
 
@@ -221,14 +221,28 @@ private function processDateTime($value)
     {
         $date = Carbon::parse($value);
 
-        return $this->formatAndCast($date, $this->saveFormat());
+        return $this->formatAndCast($this->convertTimezoneForSave($date), $this->saveFormat());
     }
 
     private function processDateTimeEndOfDay($value)
     {
         $date = Carbon::parse($value)->endOfDay();
 
-        return $this->formatAndCast($date, $this->saveFormat());
+        return $this->formatAndCast($this->convertTimezoneForSave($date), $this->saveFormat());
+    }
+
+    private function convertTimezoneForSave(Carbon $date): Carbon
+    {
+        $clientTimezone = config('statamic.system.client_timezone');
+        $appTimezone = config('app.timezone');
+        if (! $clientTimezone || $clientTimezone === $appTimezone) {
+            return $date;
+        }
+
+        // Date has the app timezone set, so we need to shift that to the CP timezone first without modifying the time.
+        return $date
+            ->shiftTimezone($clientTimezone)
+            ->setTimezone($appTimezone);
     }
 
     public function preProcessIndex($value)
@@ -244,8 +258,8 @@ public function preProcessIndex($value)
                 $value = ['start' => $value, 'end' => $value];
             }
 
-            $start = $this->parseSaved($value['start'])->format($this->indexDisplayFormat());
-            $end = $this->parseSaved($value['end'])->format($this->indexDisplayFormat());
+            $start = $this->parseSaved($value['start'], true)->format($this->indexDisplayFormat());
+            $end = $this->parseSaved($value['end'], true)->format($this->indexDisplayFormat());
 
             return $start.' - '.$end;
         }
@@ -255,7 +269,7 @@ public function preProcessIndex($value)
             $value = $value['start'];
         }
 
-        return $this->parseSaved($value)->format($this->indexDisplayFormat());
+        return $this->parseSaved($value, true)->format($this->indexDisplayFormat());
     }
 
     private function saveFormat()
@@ -346,13 +360,20 @@ public function toQueryableValue($value)
         return $this->augment($value);
     }
 
-    private function parseSaved($value)
+    private function parseSaved($value, $convertTz = false)
     {
         try {
-            return Carbon::createFromFormat($this->saveFormat(), $value);
+            $carbon = Carbon::createFromFormat($this->saveFormat(), $value);
         } catch (InvalidFormatException|InvalidArgumentException $e) {
-            return Carbon::parse($value);
+            $carbon = Carbon::parse($value);
+        }
+
+        $clientTimezone = config('statamic.system.client_timezone');
+        if ($convertTz && $clientTimezone && $clientTimezone !== config('app.timezone')) {
+            $carbon->setTimezone($clientTimezone);
         }
+
+        return $carbon;
     }
 
     public function timeEnabled()