I would use an empty `-b,--boot` option to `journalctl` in order to request "the current boot", then `-n 0` to request zero lines of output, which leaves just the header:
journalctl -b -n 0
Example output:
-- Logs begin at Wed 2021-02-10 17:46:08 PST, end at Thu 2021-02-11 15:36:01 PST. --
Or if the `-n 0` fails to output the proper information try the number 1.
~# journalctl -b -n 0
-- No entries --
~# journalctl -b -n 1
-- Logs begin at Mon 2021-02-08 20:24:14 AST, end at Thu 2021-02-11 21:33:56 AST. --
Feb 11 21:33:56 zeus-H370M systemd[1]: anacron.service: Succeeded.