There are already several useful correct answers (e.g. git log --since="6am") but it is odd that Git's special dates are missing from the documentation (at least googling "yesterday" "noon" site:git-scm.com returns no results).
it seems to accept all formats that it can output, as described in the documentation for the --date option:
--date=(relative|local|default|iso|rfc|short|raw)
Only takes effect for dates shown in human-readable format, such as when using
--pretty. log.date config variable sets a default value for log
command’s --date option.
--date=relative shows dates relative to the current time, e.g. "2 hours ago".
--date=local shows timestamps in user’s local timezone.
--date=iso (or --date=iso8601) shows timestamps in ISO 8601 format.
--date=rfc (or --date=rfc2822) shows timestamps in RFC 2822 format, often found in E-mail messages.
--date=short shows only date but not time, in YYYY-MM-DD format.
--date=raw shows the date in the internal raw git format %s %z format.
--date=default shows timestamps in the original timezone (either committer’s or author’s).
My favourite answer there is from me_and who directs us to the git date.c class. Scan down that and you find this code (at the time of writing it is on line 925):