There is a way to determine when a computer was last rebooted. This information is gathered by default, by Inventory Solution. The inventory policies that gather this information are: Collect Full Inventory and Collect Delta Hardware Inventory. You can create a gather inventory task that specifically targets this data class. This would allow you to be more aggressive reporting this data.
Open your console and navigate to Manage > Jobs and Tasks
Right click and choose New Task
On the left side, scroll down until you locate Gather Inventory and select it.
On the right side, give the new task a unique name to help you identify what it does.
Uncheck the box next to software and click on the Advanced button.
Deslect all of the available boxes and then select the Operating System data class as shown in the following screenshot:
Verify that Data table name says Inv_OS_Operating_System and select OK.
Now select OK to save the task and you're done! You can now run this against your endpoints as needed. You can then query the Inv_OS_Operating_System table for the Last Reboot Time.