The systemd journal is only transient stored in a /run/log/journal/ which is located in RAM and lost on every reboot. You can make its storage persistent on the SD Card so you can also query old boots. In /usr/share/doc/systemd/README.Debian you will find:
To enable persistent logging, create /var/log/journal:
mkdir -p /var/log/journal
Thanks above for info on dmesg and last commands. But what I found worked best was viewing var/log/messages and searching for the keyword "Booting":
less /var/log/messages | grep Booting and less /var/log/messages.1 | grep Booting.
For older archived messages, e.g., messages.2.gz, zcat /var/log/messages.2.gz | grep Booting worked nicely.
last shows a listing of last logged in users.
last -x should show what you want, although not directly.
last -x shows "the system shutdown entries and run level changes"
pi pts/0 10.1.2.100 Mon Jan 11 17:32 still logged in
pi pts/0 10.1.2.100 Sat Jan 9 18:06 - 13:08 (1+19:01)
pi pts/0 fe80::1ca4:...