You shoule be able to use
--vacuum-time. Something like the following will rotate the current active journal files into the archive and then trim all logs to keep only 10 days in that one log.
journalctl --rotate --vacuum-time=10days
Straight out of man page:
--vacuum-size=, --vacuum-time= and --vacuum-files= may be combined
in a single invocation to enforce any combination of a size, a time
and a number of files limit on the archived journal files.
Specifying any of these three parameters as zero is equivalent to
not enforcing the specific limit, and is thus redundant.
These three switches may also be combined with --rotate into one
command. If so, all active files are rotated first, and the
requested vacuuming operation is executed right after. The rotation
has the effect that all currently active files are archived (and
potentially new, empty journal files opened as replacement), and
hence the vacuuming operation has the greatest effect as it can
take all log data written so far into account.