To support the new filter code, add helper functions that turn timestamps
into year and day-of-week to core/time.c.
Internally, these functions simply call utc_mktime() to break down the
timestamp and then extract the wanted value. This may appear inefficient,
but testing shows that modern compilers are quite effective in throwing
away the unneeded calculations. FWIW in this respect clang10 outperformed
gcc10.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
No point in slurping in all of dive.h for translation units that only
want to do some time manipulation without ever touching a dive.
Don't call the header "time.h", because we don't want to end up in a
confusion with the system header of the same name.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
