You can combine multiple patterns when setting up the edit pattern to allow several different ways for entering a date.
Edit pattern to allow 08/03/2017 and 8/3/2017
break the date string first then take the month as an integer.
Compare with 12. If it is greater than 12 then it must be a date, not a month.
Then just change the format as per your need
See how to change the date format
But in your case, you can take multiple date pattern so that any date pattern will be valid to pass the validation check.