[liversage]

You have a timestamp in the future for example the time when a session at a conference starts. If to want to store this as UTC you will have to convert it from the local timezone of the conference location. You can always go back from the UTC timestamp to the local timestamp by performing the reverse conversion EXCEPT if the conversion involves daylight savings and the rule changes between now and the event in the future. While rare, daylight savings rules sometimes do change. I guess even local timezone offsets might change once in a while. When this happens the conversion back from UTC to the local time will be wrong unless you keep track of more than the UTC timestamp.