On an openSUSE system I checked the creation (aka birth) date of /var/log/wtmp will not be reset on rotation. It gets created on the first boot after installation. Thus it is likely to to show a time from shortly after the first boot started:
# ls -l --time=creation --time-style\=full-iso /var/log/wtmp
-rw-rw-r-- 1 root utmp 232323 2005-05-23 23:23:23....
Workaround is journalctl with -k, --dmesg. I am using -k as it is shorter:
It will show only kernel messages and correct time.
To show only kernel lines matching phrase:
journalctl -kg phrase