Reading converts from date-only to local time to UTC. Writing must do
the same conversion. Conversion from UTC to local time was missing,
therefore the date may have gotten shifted by a day (depending on the
local time zone).

This commit intentionally does not cache the local time zone (like
reading does), because this is a bit ugly and the assumption is that
writing occurs a lot less often than reading.

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=686693