Expiration Date safe from changing computer's date

Below is what I am using to protect my program I wrote but I know all they have to do is change the computer's date and it will work. Is there a way to protect from that?

Dim exdate As Date
    exdate = "12/31/2014"
    If Date > exdate Then
        MsgBox ("This program has ended.  Please see administrator!")
        ActiveWorkbook.Close False
        End If
        If exdate - Date < 14 Then MsgBox ("You have " & exdate - Date & " Days left. Please contact administrator.")