Systemd version: systemd-44-24.fc17.x86_64 , OS: Fedora 17.
Systemd reports wrong information about service runtime because of clock change during boot process.
[root@higgs ~]# systemctl status -- '-.mount'
-.mount - /
Active: active (mounted) since Fri, 09 Mar 2012 03:57:30 +0100; 1 years and 3 months ago
[root@higgs ~]# uptime
19:12:24 up 63 days, 22:15, 6 users, load average: 0.43, 0.30, 0.25
The system clock were off by cca a year when the system booted and this has been corrected via ntp daemon (chrony-1.27-0.3.pre1.fc17.x86_64).
I think that at least the "xxx ago" part of timing information should not be affected by clock changes the same way uptime isn't.
Running systemd on a RaspberryPi (no RTC available), systemctl status reports it is running over 44 years:
# systemctl status -- '-.mount'
● -.mount - /
Loaded: loaded (/etc/fstab)
Active: active (mounted) since Thu 1970-01-01 01:00:03 CET; 44 years 3 months ago
Warning: Journal has been rotated since unit was started. Log output is incomplete or unavailable.
21:48:02 up 13 min, 1 user, load average: 1.06, 0.98, 0.63