mktime() is supposed to fill in tm_wday and tm_yday, and it
it's supposed to canonicalize out-of-range values (that is,
"Jan 31" is turned into "Feb 1").
Instead of making the straightfoward tm_to_time() implementation
more complicated, just make it call time_to_tm() once the timestamp
is computed to break it back down ot (canonical) tm entries.