not clean yet

app/Http/Controllers/Gallery/PhotoController.php

@@ -141,13 +141,21 @@ public function update(EditPhotoRequest $request): PhotoResource
 		$photo->tags = $request->tags();
 		$photo->license = $request->license()->value;
 
-		if ($request->takenAt() !== null) {
-			// update if required
-			$photo->taken_at_mod = $request->takenAt();
-		} elseif ($photo->taken_at_mod !== null) {
-			// reset if set to null
+		// We only use taken_at_mod to store the original date when the user changes the date.
+		// If the user changes the date again, we do not update taken_at_mod.
+		if ($request->takenAt() !== null && $photo->taken_at_mod === null) {
+			// First time saving: we preserve the original date in taken_at_mod.
+			$photo->taken_at_mod = $photo->taken_at;
+			$photo->taken_at = $request->takenAt();
+		} elseif ($request->takenAt() !== null && $photo->taken_at_mod !== null) {
+			// We already have a taken_at_mod, we only update the taken_at.
+			$photo->taken_at = $request->takenAt();
+		} elseif ($request->takenAt() === null && $photo->taken_at_mod !== null) {
+			// we reset
+			$photo->taken_at = $photo->taken_at_mod;
 			$photo->taken_at_mod = null;
 		}
+
 		$photo->save();
 
 		return PhotoResource::fromModel($photo);

app/Http/Resources/Models/PhotoResource.php

@@ -88,7 +88,7 @@ public function __construct(Photo $photo)
 		$this->shutter = $photo->shutter;
 		$this->size_variants = new SizeVariantsResouce($photo);
 		$this->tags = $photo->tags;
-		$this->taken_at = ($photo->taken_at_mod ?? $photo->taken_at)?->toIso8601String();
+		$this->taken_at = $photo->taken_at?->toIso8601String();
 		$this->taken_at_orig_tz = $photo->taken_at_orig_tz;
 		$this->title = $photo->title;
 		$this->type = $photo->type;