How to find "how long the system has been up" .....
regardless of the system localtime
The methods I know rely on comparing the date/time that the system started to another date/time. Only root can modify the date on my systems so I trust the value returned.
You could use an external box. For example, when the system boots it logs to (or emails) an external trusted system. Your calculations of uptime are done on this box instead. It's not very elegant though.