“I want the date on my computer to be six months in the past, instead of today’s date.I tried changing the date, but it keeps going back to today’s date. Is there a way I can backdate my computer?”
By default, Windows automatically updates date and time information based on time servers on the Internet. This means that when you set you computer date to a point in the past, the next time the automatic update check takes place, your computer will be reset to the correct time. The good news is you can turn off automatic time updates.

As you already discovered, setting the clock back on your computer will only work for the time between automatic updates. If you want to keep your computer set to a time in the past, you need to disable automatic updates to your computer’s clock. To do this click on the time currently displaying in the Windows Taskbar, then click on change date and time settings.
From here, you want to click on the Internet Time tab. This is what Windows uses to automatically update the clock on your computer. By turning it off, you have control to manually set the time as you see fit. Click the Change Settings button on this tab.

The last step is to uncheck the box to Synchronize with an Internet time server and click OK. From here you will be able to set your computer to whatever time you would like and it will stay at that time until you change it back. If you want to reset your computer to the correct time, the easiest thing to do is reverse the steps here and automatically update your computer’s time.