An important concept to have in your mind when dealing with timezones is that "Time Zones" and "Zone Identifiers" are not the same thing. "Time Zones" refer to

An important concept to have in your mind when dealing with timezones is that "Time Zones" and "Zone Identifiers" are not the same thing. "Time Zones" refer to an offset from UTC used to compute the local time; "Zone Identifers" identify a geographic region that has political rules about what its current time zone is. For example, "America/New_York" is a standard identifier for a geographic area. Depending on the date, it is either 4 hours or 5 hours behind UTC. That is what we call daylight savings time, and it controls whether America/New_York uses the "Eastern Standard Time" (EST; GMT-5) or the "Eastern Daylight Time" (EDT; GMT-4) time zone. If those political rules change, and you threw away the local time zone info as it existed when the record was created (because your app converts everything to UTC) then users will get the wrong times for their events.