In order to get boots list on Ubuntu (generally on linux), we can use this command
journalctl --list-boots
and see the timestamp, similar to below snapshot.
-5 8c9ec77f40xxxxxxxxxxxxxxxxxxxxxx Thu 2023-07-13 12:48:54 UTC—Thu 2023-07-13 12:48:55 UTC
-4 3a489cee09xxxxxxxxxxxxxxxxxxxxxx Fri 2023-11-03 15:21:12 UTC—Mon 2023-11-06 08:57:08 UTC
-3 470525945cxxxxxxxxxxxxxxxxxxxxxx Mon 2023-11-06 08:58:12 UTC—Fri 2023-12-01 15:04:03 UTC
-2 520a716e66xxxxxxxxxxxxxxxxxxxxxx Fri 2023-12-01 15:05:15 UTC—Mon 2023-12-04 19:21:17 UTC
-1 1ab06ba385xxxxxxxxxxxxxxxxxxxxxx Mon 2023-12-04 19:22:28 UTC—Tue 2023-12-12 11:54:17 UTC
0 63920e3ee7xxxxxxxxxxxxxxxxxxxxxx Tue 2023-12-12 11:55:22 UTC—Sat 2023-12-23 00:11:02 UTC
Note that current boot is the last line, indexed as 0.