Sounds like the quoted RFC would help here. Storing the offset would make it unambiguous which of the two moments was meant. Your business logic would have to figure out what to do when the offset no longer exists (honour the clock time or convert to the new timezone) or is nonsense. The geographical reference would help decide what to do if you're not in a single location.
Is this an improvement on how long whatever it's talking about usually gets updates for, or is putting a limit on it at all a bad sign? I've only seen this with regards to mobile phones before.