If we call time(NULL), then do something (however trivial), then call
g_date_time_new_now_utc(), they do not necessarily share a seconds
value. Let's say the gmtime call takes 2ms. time(NULL) could
return xx:xx:23 when the time is actually xx:xx:23.999999, resulting
in the g_date_time_new_now_utc() happening at xx:xx:24.000001. This is
unlikely, but did happen to me in a parallel build:

GLib:ERROR:.../glib/tests/gdatetime.c:674:test_GDateTime_now_utc: assertion failed (tm.tm_sec == g_date_time_get_second (dt)): (23 == 24)

A similar argument applies to the rollover from xx:23:59.999999 to
xx:24:00, so comparing seconds with a 1s "fuzz" or a >= comparison
is not sufficient; and so on into higher-order fields.

I haven't seen the other tests that use _now() fail in the same way,
but they could.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=749080


Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>