OID used by NPM to determine system has rebooted?

yes.

Use the following alerts and UnDP if you want to use the HOST-RESOURCES MIB variables instead (which indicate the ACTUAL system reboot, rather simply the restart of the SNMP agent)

hrSystemReboot.AlertDefinition

/RjL

Thank you Richard. One question please. The second trigger condition is "Rate is less than 60000". What exactly does that mean?

60,000 centiseconds [the standard time unit in SNMP for some strange reason, quite why they didn't pick a standard SI prefix is beyond me]

Anyway 600 seconds or 10 minutes: has the node rebooted in the last 10 minutes.

/RjL

Yes, and I then tested the OID against a server I rebooted manually. The funny thing is, the number does not make sense unless you calculate a "time tick" as 1/1000 of a second, rather than 1/100. In other words, if I test the OID every 5 seconds and look at the output, it is definitely coming back in thousandths of a second.

It should be TimeTicks (CentiSeconds), at least that is what the MIB says it should be:

I'm unsurprised by software writers being unable to interpret MIBS correctly: if your system returns it in thousands of a second adjust accordingly (and file a bug-report with the vendor).