Windows server uptime resets when snmp service on server is restarted

Hi,  I have an alert that says if the device_uptime is less than 360, alert that the server was down.  I happened to restart the snmp service on several of my servers and was surprised to find the alert triggered.  I am guessing that when snmp restarts, the uptime counter goes to zero?  Is there any way to get a true read of the uptime?  I am trying to alert if the server rebooted within the poling window.

thanks

Tony