To see information from your most recent boot, run
To see the details that were logged during a previous boot, just in case you need to compare them with your most recent, run
You will then see results similar to the output shown below, but will need to page to the bottom of the output to see the most recent.
-2 5d45a92d9d5a42d8ae1c8b454f7e1314 Sun 2018-08-19 02:29:21 PDT—Sun 2018-08-19 02:36:26 PDT
-1 d5b8ebc236734ee69c48c3809ae452b1 Sun 2018-08-19 02:43:06 PDT—Sun 2018-08-19 07:20:54 PDT
0 6f844406186e4e31ba0375a7bc736242 Sun 2018-08-19 10:43:31 PDT—Sun 2018-08-19 18:54:51 PDT
At the very bottom of the output, you will see a line that starts with 0, which will represent your most recently logged boot info, the same info you saw when you ran journalctl -b.
To see the data that was logged during the previous boot, copy the identifier from the line that starts with -1 and run the following
To go back further in time, just change to -2, -3, -4, etc.