[AccessD] Check dateformat in wich event?

hi group,
 
I need to check a dozen date-fields and check if the date entered is DD-MMM-YYYY.
I've created a constant:
Public Const DATE_FORMAT As String = "dd-mmm-yyyy"
 
I want to check the date format like this:
Me.txtDate.Text = Format(Me.txtDate, DATE_FORMAT) 

Wich event is most suited for this?
 
TIA
 
Sander


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