Converting time from timezone string to another timezone time

[Jeffrey Walton]

Hi Everyone,

I'm having a heck of a time converting a time from a timezone string
to another timezone time. The best I seem to be able to do is adjust
for DST using strptime, mktime and localtime. For example:

  * given: '15 Jan 2021 01:24:55 -0800 (PST)'
  * expected: '15 Jan 2021 04:24:55 -0500 (EST)' (or similar, as long
as it includes 04:24:55)
  * my best result: '15 Jan 00:24:55 2021' (seems to be a DST adj of PST time)

I've been through the manual at
https://www.gnu.org/software/libc/manual/html_node/Calendar-Time.html
but I can't seem to find the sequence of calls to perform the actions.
I've also been through the libc FAQ at
https://sourceware.org/glibc/wiki/FAQ (there's one topic on
timezones).

So I have two questions, but I only need one answered (whichever is
easier to answer):

1. Where is the discussion/faq on how to convert from timezone string
to another timezone time?

2. What is wrong with this code (error checking omitted)?

    const char pst_timestring[] = '15 Jan 2021 01:24:55 -0800 (PST)';
    struct tm gmail_tm[1];
    strptime(pst_timestring, "%d %b %Y %T %z", gmail_tm);

    // Already set, but just in case. It is -0500 (EST)
    char my_timezone[] = "TZ=America/New_York";
    putenv(my_timezone);

    time_t utc_time;
    utc_time = mktime(gmail_tm);

    struct tm local_tm[1];
    local_tm = localtime(&utc_time);

    // This prints a time that was adjusted by 1 hour, not 3 hours
(expected from -0800 to -0500)
    printf(..., asctime(local_tm));
    // Output is similar to '15 Jan 00:24:55 2021'

Somewhere there is a non-obvious problem with the code stitched
together from the man pages.

Thanks in advance.

Jeff