If you can not double-click on the clock in the system tray, try the following steps:
1: Click Start Button
2: Open Control Panel
3: A) Click on "Date, Time, Language, Regional Options"
or B) if you are in "classic mode" click on "Date and Time" option and skip to step 5.
4: Click on "Change Date and Time"
5: Edit the date and time as you see fit, and click apply.
I am not sure how you are trying to access the date/time adjuster, But try these steps (if you have not already)