changing of date format

Just to add to what Marcus says, conversion from a string to DateTime should be done as early as physically possible, and conversion from a DateTime to a string should be done as late as possible.

The reasons for this are to do with localisation - the earlier you convert from a string, the more likely you are to be able to access the locale information that the user has set, and the more likely you are that the conversion will be correct and the end result what the user intended to enter.
For example, if the user enters 10/11/12 does he mean 10th November 2012, 11th Oct 2012, or 12th November 2010? If you have access to the locale information you can tell and parsing of the string can be done correctly.

The later you convert back to a string, the more likely you are to generate an understandable date fro teh user - who may have a totally different locale and date format.

Remember that DateTime does not store a date or time as such - it stores it as an interval in milliseconds since a fixed point in time. So conversion to or from a string with reference to the locale the user is expecting means that date can be correct even if the dating system is not Gregorian - Hirji for example uses the lunar year in which Muhammad immigrated from Makkah to Madinah.
The Gregorian date for today 25 Oct 2012 is (approximately) 9 Thw al-Hijjah 1433 in Hirji.
Converting as I describe means this doesn't have to complicate your software at all.