Template:FAQ:Agent 44

Oh no it's not. The defined meaning of 'sysUpTime' is

       "the time ... since the network management
        portion of the system was re-initialized."

In other words, when the snmp agent was started, not when the system itself last booted. This latter information is available in the Host Resources MIB as "hrSystemUpTime.0". Note that even if the full Host Resources is not supported on your system, it's worth configuring in the system portion using

               '--with-mib-modules=host/hr_system'

and recompiling. This particular group is reasonably likely to work, even if some of the other more architecture-specific groups don't.