Users are expected to be familiar with git's "approxidate"
functionality for parsing dates, so we'll expose that
in our UIs. Xapian itself has limited date parsing functionality
and I can't expect users to learn it.
This takes around 4-5ms on my aging workstation, so it'll
probably be made acceptable for the WWW UI, even.
libgit2 has a git__date_parse function which I expect to have
less overhead, but it's only for internal use at the moment.