List boots with unix timestamps via journalctl

journalctl has the -o short-unix flag that I can use to change the output date format on stuff like -t systemd-sleep. But the only way I've found to list boots is --list-boots, and this doesn't seem to obey the -o flag. Is there a way to make journalctl list boots with unix timestamps? Since systemd is here to stay I fear other methods might break in the future, but I'm open to those suggestions too.