Server-side validation w/@Formula: 
Following is example of input validation event for a TEXT field (not a date field) for when you want to store the date as text but still validate the structure / format of the date entered. 
 
  
 
Client-side validation w/JavaScript: 
 regex_date_validation.rtf 
I believe this reg-ex also handles leap year, function is expecting western date format of mm/dd/yyyy. 
  
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