Hi,
I think that your approach could be a much simpler one. We have to realize that on the variable pop-up, the date is displayed in "external format" (prerequisite is that your input field is type date). The display is dependent on the standard SAP "date format" setting in the user's Defaults.
Please have a look in t/code SU3, second tab page Defaults. There is a field called "Date Format". Behind this field is domain XUDATFM. It can have different values depending on the user's preference. It can of course also be set by an administrator, e.g. via t/code SU01 or a corresponding CUA t/code.
In my user it is set to DD.MM.YYYY (i.e. domain value '1'). But it can also be set to YYYY/MM/DD (Iranian Date) (i.e. domain value 'C').
The "internal date format" is however always in Gregorian date format. So in my opinion the conversion from external format to internal format should be done automatically by SAP.
I suggest to remove the user-exit variable and use the date input variable for restricting the selection.
Best regards,
Sander