Re-entrant, any-zone mktime()-equivalent?

Paul Eggert eggert at
Thu Nov 16 02:35:54 UTC 2000

> From: Jonathan Lennox <lennox at>
> Date: Wed, 15 Nov 2000 17:41:18 -0500 (EST)

> Has anyone ever developed patches for tzcode to support a function that is
> the equivalent of timelocal()/mktime() and timegm() for arbitrary time zones?

Not that I know of.  The subject has come up more than once, but
nobody has done it as far as I know.

There is some Java code to do it: see 

You might also consider starting from the GNU C library code base.
There is a serious (i.e. it will probably be implemented soon)
proposal to do thread-safe locales with glibc in
The same basic idea applies to time zones.
The glibc maintainer might buy back changes along these lines.

> <>.)

>From the tz list's point of view, Appendix A of that draft is perhaps
the most interesting one.  But frankly I couldn't follow it -- I have
a vague impression of what it's doing, but I couldn't make heads or
tails of it really, even though I read the earlier part of the RFC.
Some examples would help immensely.

I did notice this point:

        1.   Compute the time of the call, in the timezone of the time
             switch.  (No step after this needs to consider time zones
             -- all calculations are done using continuously-running
             standard Gregorian time.)

Suppose the current time zone is America/Los_Angeles, the time of the
call is 2000-04-02 01:30 -0800 (PST), and you have a two-hour time
slot.  (As I don't follow your terminology, I'm using informal terms
like "time slot".)  Since you use standard Gregorian time, I guess
you'll say that 2000-04-02 03:45 -0700 falls within the two-hour time
slot.  But 03:45 actually is only 1 hour and 15 minutes after the time
of the call, due to a DST change.  Is that what the user wants?

As an extreme case, consider Pacific/Kiritimati.  In that time zone
there is no day 1995-01-01, as they moved the clock 24 hours ahead at
midnight.  What will Algorithm A do with that case?  There are also
cases of clocks moving backwards 24 hours.

It might be helpful for you to supply examples that cover cases like

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