journalctl --list-boots
will show all the boots and their relative numbers and start times that are currently in the journal logs.
Also, when you do things such as login/logout you can note the time and then use the -S (or --since) and -U (or --until) to narrow the output of journalctl to keep the amount of captured info to a minimum and to the time relevant to events you're interested in. That would eliminate the guesswork of others who may decide to look at your logs. a.k.a. making life easier on others.