The following notice I am getting in /var/adm/messages file
Dec 9 06:17:24 hostname lw8: [ID 925147 kern.notice] Main, up 476 days 04:42:23, Memory 9,742,808
Dec 9 10:17:25 hostname lw8: [ID 433305 kern.notice] Main, up 476 days 08:42:23, Memory 10,179,864
There are certainly known issues in Open Solaris if your Real Time Clock has a flat battery (or was wrong at startup) and the clock was subsequently corrected manually or corrected with NTP.
i.e. The clock changed after the kernel loaded but before System Accounting made a timestamp in the "utmp" file (which is used by the "uptime" command).