AX: VoiceOver reports the datetime value in the wrong time zone.

https://bugs.webkit.org/show_bug.cgi?id=269999
<rdar://problem/123522296>

Reviewed by Chris Fleizach.

VoiceOver expects the value of dateTime elements to be in local time zone. Since we store those values in GMT, we need to convert them to local times before returning them.

* Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperMac.mm:
(gmtToLocalTimeOffset):
(-[WebAccessibilityObjectWrapper accessibilityAttributeValue:]):

Canonical link: https://commits.webkit.org/275265@main

Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperMac.mm

@@ -1511,6 +1511,14 @@ - (NSArray *)_transformSpecialChildrenCases:(RefPtr<AXCoreObject>)backingObject
     return nil;
 }
 
+// VoiceOver expects the datetime value in the local time zone. Since we store it in GMT, we need to convert it to local before returning it to VoiceOver.
+// This helper funtion computes the offset to go from local to GMT and returns its opposite.
+static inline NSInteger gmtToLocalTimeOffset()
+{
+    NSDate *now = [NSDate date];
+    return -1 * [[NSTimeZone localTimeZone] secondsFromGMTForDate:now];
+}
+
 // FIXME: split up this function in a better way.
 // suggestions: Use a hash table that maps attribute names to function calls,
 // or maybe pointers to member functions
@@ -1705,8 +1713,10 @@ - (id)accessibilityAttributeValue:(NSString*)attributeName
             [] (float& typedValue) -> id { return @(typedValue); },
             [] (String& typedValue) -> id { return (NSString *)typedValue; },
             [] (WallTime& typedValue) -> id {
-                RetainPtr date = [NSDate dateWithTimeIntervalSince1970:typedValue.secondsSinceEpoch().value()];
-                return date.autorelease();
+                NSInteger offset = gmtToLocalTimeOffset();
+                auto time = typedValue.secondsSinceEpoch().value();
+                NSDate *gmtDate = [NSDate dateWithTimeIntervalSince1970:time];
+                return [NSDate dateWithTimeInterval:offset sinceDate:gmtDate];
             },
             [] (AccessibilityButtonState& typedValue) -> id { return @((unsigned)typedValue); },
             [] (AXCoreObject*& typedValue) { return typedValue ? (id)typedValue->wrapper() : nil; },