_GIT_SINCE defaults to 2005-04-07, which is surprising
(for instance, GNU MPFR started in 1999).
Indeed, in the source:
# Beginning git log date. Respects all git datetime formats
# If $_GIT_SINCE is never set, choose epoch time as that is
# as far back as git will allow you to go
_since=${_GIT_SINCE:-}
if [[ -n "${_since}" ]]; then
_since="--since=$_since"
else
_since="--since=2005-04-07"
fi
But the epoch time is 1970-01-01, not 2005-04-07. It would be
better not to have a default limit, but I doubt that there are
git repositories with dates before 1970...