How to get the current time in a different city in C?

Question

I'm writing a server in C that is meant to get a request over UDP for the time in either: Doha, Prague, New York or Berlin. How can I produce the right response?

I tried looking into the localtime() method but couldn't find anything relating to a timezone

you can check [here](https://en.cppreference.com/w/c/locale/LC_categories) — sinanorl, Dec 16 '22 at 07:16
This looks like it's only taking care of the formatting, not the actual timezone. Are you sure this'll work? — Elad Moshe, Dec 16 '22 at 07:39
"New York" is not a _capital city_. It is a major city in its time zone. — chux - Reinstate Monica, Dec 16 '22 at 12:02

Elad Moshe · Answer 1 · 2022-12-16T12:58:50.517

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Ok so I figured it out on my own. Let's say I want the current time in Doha. Apparently Doha's time is UTC+3. Which means that I need to get the current UTC time and add 3 hours to it like so:

char* response = malloc(255 * sizeof(char));
time_t now = time(NULL);
tm* t = gmtime(&now); //Gets me the UTC time
t->tm_hour += 3; //Adds 3 hours to it
mktime(t); //Normalizes it (so it's 60 seconds in a minute, 60 minutes in an hour and so on...)
strftime(response, 255, "%H:%M:%S", t);

edited Dec 16 '22 at 12:58

answered Dec 16 '22 at 08:04

Elad Moshe

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Does this work when tm_hour is 23? – pmacfarlane Dec 16 '22 at 08:07
Only if you use mktime on it. It'll normalize it. – Elad Moshe Dec 16 '22 at 08:10
What is `new` in this C question? – chux - Reinstate Monica Dec 16 '22 at 12:03
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er, you need to handle DST, and that is set by psusdo-random government decree – Andrew Dec 16 '22 at 12:34
Ah sorry, I'll edit new to malloc – Elad Moshe Dec 16 '22 at 12:57
In the case of DST, I suppose you can do t->tm_isdst = 1 and mktime(t) will change it accordingly but I'm not sure. – Elad Moshe Dec 16 '22 at 13:04
@EladMoshe That's `tm_isdst = -1`, not 1. See also [this question](https://stackoverflow.com/questions/74756775). – Steve Summit Dec 16 '22 at 13:32
Careful with assumptions about fixed offsets. That's what the TZDB is for. Watch: https://www.youtube.com/watch?v=-5wpm-gesOY – Matt Johnson-Pint Dec 16 '22 at 17:32
Even Doha wasn't always UTC+3. It used to be UTC+4. The switch happened in 1972. See the chart on https://www.timeanddate.com/time/zone/qatar/doha?syear=1960&eyear=2030 or the source data in the tzdb here: https://github.com/eggert/tz/blob/b2e7a66b74c5b7a94c91d3e9f131b02199c9dd0b/asia#L3520-L3525 – Matt Johnson-Pint Dec 16 '22 at 17:34

How to get the current time in a different city in C?

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