Try this way,
Str variable = DateTime.ParseExact("Yourinput",{"dd-MM-yyyy","dd-MMM-yyyy","dd-MMMM-yyyy","d/M/yyyy"},System.Globalization.CultureInfo.InvariantCulture,System.Globalization.DateTimeStyles.None).ToString("dd.MM.yyyy")
If you have more input format means you can add in the array
Regards,
Gowtham K