Best way to convert Unix time_t to/from abitrary timezones?

Question

My code receives a time_t from an external source. However, that time_t isn't acutally based on UTC Epoch time, along with it I get a timezone string (eg, EDT, PST, etc), and its based on this offset'ed epoch. I need to convert this to a true UTC Epoch based time_t.

Additionally, I need to be able to go in the opposite direction, taking a UTC Epoch based time_t and a timezone, and create the offsetted time_t, but in this situation instead of having a timezone like EDT/PST), etc, I have a Unix style timezone description like "America/New York" and need to convert to the correct timezone given daylight savings, so I'd need to get back from the algorithm, both an offsetted time_t, and the correct descriptor (EDT,EST).

I'm pretty sure I can pull this off by temporarily changing tzset() and some combination of conversions between broken-down time, time_t and timeval, but doing this always makes my brain hurt and makes me feel like I'm missing something obvious. Can anyone recommend some code or sudo-code to do this or at least a proper approach?

Answer 1

would any of these functions help you? localtime(), gmtime(), etc.

Answer 2

time_t is in seconds, so just offset your time_t values by 3600 times the number of hours the timezone is offset by. As long as you have the offset specifically identified (i.e. EST or EDT instead of US/Eastern or EST5EDT or whatnot) then this is really simple and not prone to errors.