You do not have proper privilege level to change the system time

What operating system?

Log on using an administrators account.

If your on a networked environment and not an admin then you won't be able to change it, if this is a personal PC and your under the administrator account and still can't change it then you have a virus. In that case I suggest the following downloading Avast, it is a free virus scanner and the best on the market free or paid for. Also download malewarebytes and adaware to scan your computer with.