Try this. Doues it work for your needs?

    txtDate = Null
    txtDate.SetFocus
    txtDate.SelStart = 0

txtDate = Null will clear the date already in the field.
txtDate.SetFocus will set the focus back on the field.
txtDate.SelStart = 0 will place the cursor at the start of the Input Mask and display it.

Hi Andrew, do you want to clear the field or undo the user entry?  Your code indicates that you want to undo, and that is done differently.  If the Undo function "isn't available," it needs to be done some other way, such as using the .OldValue property.