systemd is the modern init system used on many Linux distributions, including Ubuntu. The command to query systemd's journal is journalctl. Querying the journal is useful to find out about previous boots and to look at logs of what apps went wrong.
$ journalctl --list-boots
This can be useful to see your previous uptimes.
$ journalctl --pager-end
$ journalctl --boot=X
Tried with: Ubuntu 18.04