don't use mktime() shortcut in _datetime_normalize()
	arg_tm in datetime_parse() is currently incorrect after normalization
	this fixes that and avoids assuming that time_t is in single seconds
	use difftime()
	don't clobber now_tm
datetime_parse()'s third arg should be a (struct tm *)
if -d, printf() parsed time, not diff from epoch
drop timegm()
try to fix dst bug
test with other libc's (newlib? musl? fbsd? obsd?)
`./@ 7:00 PM` doesn't work with musl strptime()
test other times
distcheck
gettext?