64-bit Unix timestamp conversion

You need:

typedef long long time64_t; 
time64_t mktime64(struct tm *t); 
struct tm* localtime64_r(const time64_t* t, struct tm* p);

Originally (in 2011) this answer contained links to 2038bug.com where it was possible to download the small pivotal_gmtime_r library, containing the mentioned functions. The library has been removed from 2038bug.com back then, the links became broken and were removed from the answer by a moderator. Seems like that pivotal_gmtime_r code can now be found here:

https://github.com/franklin373/mortage/tree/master/time_pivotal

Also, I've found another, more recent library, called y2038, that also implements mktime64 and localtime64_r:

https://github.com/evalEmpire/y2038

The function converting a struct tm* to a time_t is mktime. You can find many implementations of it, eg. in Glibc and in libvxc's mktime.c file. You could take the code (assuming it is legal to you, so please respect licenses) and change time_t to some 64 bits integer like int64_t.

The functions doing the other conversions from time_t to struct tm* are localtime or gmtime and you could do likewise.

However, you might have a more fundamental issue: your 32 bits machine running in the year 2040 should have some way of giving you the current time (as the time system call does) appropriately in the 64 bits variant of time_t, and that is much harder (it depends upon the kernel and the hardware).

You seem to be making the assumption that time_t is 32-bits on 32-bit systems, and this may or may not be true.

On Windows, starting with Visual Studio 2005 the size of time_t is 64-bits, even when you compile for 32-bit Windows.

The unfortunate part is that glibc defines it as long int, which on 32-bit systems is a 32-bit integer. That means that 32-bit Linux and other 32-bit platforms that are based on gcc/glibc (like Cygwin) will not be able to work with 64-bit timestamps.

If your application must run on 32-bit glibc, then you should use your own conversion functions, which could be the same functions in the C library recompiled to use 64-bit timestamps.

If you need source code with a permissive license (BSD), then you can look at these functions in minix3. Here is localtime. The source is hyperlinked, so you can find the others easily.