In the right-down corner of the screen in the taskbar notification area click the Clock
in the dialog window that will open click the Change date and time settings... link
in the Date and Time window click the Change date and time button
in the Date and Time Settings window set the correct date and click the OK button
You can find detailed instructions on how to change the system date on your computer and synchronize the time by using the Windows Time Service in the article Set the clock of Microsoft Knowledge Base.