Question

In .NET, most of the standard strings used for formatting a DateTime value are culture-aware, for example the ShortDatePattern (“d”) format string switches the order of the year/month/day parts around depending on the current culture:

6/15/2009 1:45:30 PM -> 6/15/2009 (en-US)
6/15/2009 1:45:30 PM -> 15/06/2009 (fr-FR)
6/15/2009 1:45:30 PM -> 15.06.2009 (de-DE)

I need something similar for a date format containing only month and day:

6/15/2009 1:45:30 PM -> 6/15 (en-US)
6/15/2009 1:45:30 PM -> 15/06 (fr-FR)
6/15/2009 1:45:30 PM -> 15.06. (de-DE)

Using e.g. the “MM/dd” custom format string doesn’t work; it will incorrectly display “01.11” for Jan 11 in the German culture, when I want it to display “11.01.”

How can I build a custom format string that takes the order of the date parts into account?

Answer 1

Assuming all cultures passed will have a ShortDatePattern that contains M or MM and d or dd in some order with some sort of separator (I can’t think of an exception for the Gregorian calendar, but I could be wrong) then the following will work:

private static string FindMonthDayOnly(System.Globalization.CultureInfo ci)
{
  string shortPattern = ci.DateTimeFormat.ShortDatePattern;
  while(shortPattern[0] != 'd' && shortPattern[0] != 'M')
  {
    shortPattern = shortPattern.Substring(1);
    if(shortPattern.Length == 0)
      return ci.DateTimeFormat.ShortDatePattern;
  }
  while(shortPattern[shortPattern.Length - 1] != 'd' && shortPattern[shortPattern.Length - 1] != 'M')
  {
    shortPattern = shortPattern.Substring(0, shortPattern.Length - 1);
    if(shortPattern.Length == 0)
      return ci.DateTimeFormat.ShortDatePattern;
  }
  return shortPattern;
}

If the assumption is very far mis-matched, then it’ll revert to ShortDatePattern, though it could also end up showing just the date or just the month.

It fails in not correctly including separators at the end or other indicators when appropriate. For example, it turns the standard format (yyyy-MM-dd) into MM-dd rather than --MM-dd, which is the standard format for month-day combinations with no year.