Checking Date in Macro Code

I have a spreadsheet that I created that we use as our Invoice. I have created a macro button that does certain checks/balances when it is pressed, after the invoice has been created. One of the common problems was that the employees would forget to put a date on the invoice. So now, I have the code for that:

If Range("D5").Value = "" Then
     Range("D5").Select
     If MsgBox("You Forgot to Enter Date!", vbCritical) = vkOK Then Exit Sub
End If

But now, I want to go one step further. In addition to checking to see if there is somthing in the field, I want it to check to make sure that the date entered is TODAY and if it's not, to popup with a MSGBOX that says, "Are you sure you want to enter a date other than today?". If they enter yes, the code proceeds with the checking. If they enter No, I want the program to stop running and to go to that field D5 and wait for the user to correct the date. As a newbie, I'm just unclear what function to use. I've tried various things, all to no avail. Can anyone help me? Thank you!