Limited Time Gifts are timed and required to be logged in within a certain timeframe.
However, if you enter the game upon receiving the notification and see no gift, you could be experiencing the issue below.
These gift notifications are based on what you have set your device's local time to. However, the gifts will be sent based on what your DSA account's time zone is.
These two can be different, especially if you've moved since you started playing DSA.
If you get a notification and don't see your limited-time gift in your inbox, please send in a support ticket regarding the limited-time gift and also let us know what your current time zone is so we can investigate and help you out.