Here i have attached UnDP poller for all windows and Linux servers to get exact last reboot time. By default solarwinds is showing last SNMP restart time in Node details view.
Using this poller you can get correct reboot time with graph also. Apply this poller to any node(windows/Linux).
after assigning just click on Gauge to see graph for all reboot history.(after next reboot)
You can find report for the same.