Windows automatically resets your computer's uptime -- the total number of hours that your computer has been running since its last reboot -- each time you restart the machine. This means that although you cannot track the total number of hours your computer has been running since it was first powered on, you can still determine how long it has been running since its last reboot. You can view your computer's current uptime by launching the Task Manager utility. If you prefer to calculate your computer's uptime manually, you can access the date and time the machine was last restarted in the Event Viewer utility and by using the systeminfo command.
Task Manager Displays Current Running Time
Right-click the "Start" button.
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Choose "Task Manager" from the menu and then select the "Performance" tab.
Click "CPU" to view the current uptime in the Up Time section.
Use Command Prompt to Find System Uptime
Press the "Windows" key to open the Start screen and then type "cmd."
Select "Command Prompt" from the Search bar.
Enter "systeminfo" (without quotation marks) in the Command Prompt window and then press "Enter." The command displays information about your operating system and computer. The last system boot date and time is listed next to "System Boot Time."
Subtract the last system boot date and time from your current date and time to find the total hours your computer has been running since the last system boot.
Find the Elapsed System Time With Event Viewer
Press the "Windows" key and then type "Control Panel."
Select "Control Panel" from the Search bar and then select "System and Security."
Select "Administrative Tools" and then double-click "Event Viewer."
Double-click "Windows Logs," right-click "System" and then select "Filter Current Log."
Enter "6005" in the "All Event IDs" field and then click "OK."
Click "System" under Windows Logs. The first entry in the upper middle pane of the window is the date and time of the last system boot. Subtract this date and time from the current date and time to get the total elapsed time from the last system boot.