It actually is especially difficult at scale for non-trivial applications. Large databases won't be able to atomically change every instance which may lead to v

It actually is especially difficult at scale for non-trivial applications. Large databases won't be able to atomically change every instance which may lead to very frustrating bugs where collisions cannot be allowed. For example, on a calendar app. I agree with dtech, if you're storing in the future don't use a timestamp unless the input value is specifically intended to be timezone unaware, which is typically only useful for short term things like control systems or alarms. For longterm human uses a datetime+timezone field or an ISO datetime+timezone string is safer.