Rolling dates by pressing "+" or "-"

Using Windows Vista & Access 2007

The following is taken from:  Allen Browne's tips for Microsoft Access

Rolling dates by pressing "+" or "-"
Some commercial programs (Tracker, Quicken, etc) allow the user to press
"+" 
or "-" to increment or decrement a date without the hassle of selecting
the 
day part of the field and entering a new value. This is especially useful
in 
fields where the default date offered could be a day or two different from

the date desired.
To provide this functionality in Access, attach this Event Procedure to
the 
KeyPress event of your control.

Select Case KeyAscii
    Case 43            ' Plus key
        KeyAscii = 0
        Screen.ActiveControl = Screen.ActiveControl + 1
    Case 45            ' Minus key
        KeyAscii = 0
        Screen.ActiveControl = Screen.ActiveControl - 1
 End Select

I've followed the instructions above, but I get an error at the
"KeyAscii."  
The Error Message says that the Variable KeyAscii not declared.
Does this indicate a missing reference, and if so which one; or is there 
something else I'm missing?

Thanks for any and all assistance.
Byron