"Even if the time is manually configured, the on-board clock will still drift."
Yea, system devices that keep poor time have always, somewhat, amazed me because I've always wondered why they cannot, if powered by public AC, "count" that (which tends to be very, very precise - how often does your cheap AC clock need time adjustment). Of course, as you (and others) also noted, there's NTP.
In any case, some Cisco devices, with an on-board clock and can also occasionally (if configured) also sync their on-board clock with NTP. Then if you lose NTP access, or during the initial boot (before system reacquires NTP sync), timestamps will often be fairly close to correct time.
