DateTime is always in the Gregorian calendar, effectively. Even if you create an instance specifying a dfferent calendar, the values returned by the Day, Month, Year etc properties are in the Gregorian calendar.
As an example, take the start of the Islamic calendar:
using System;
using System.Globalization;
class Test
{
static void Main()
{
DateTime epoch = new DateTime(1, 1, 1, new HijriCalendar());
Console.WriteLine(epoch.Year); // 622
Console.WriteLine(epoch.Month); // 7
Console.WriteLine(epoch.Day); // 18
}
}
It's not clear how you're creating the input to this method, or whether you should really be converting it to a string format. (Or why you're not using the built-in string formatters.)
It could be that you can just use:
public static string FormatDateTimeAsGregorian(DateTime input)
{
return input.ToString("yyyy'/'MM'/'dd' 'HH':'mm':'ss",
CultureInfo.InvariantCulture);
}
That will work for any DateTime which has been created appropriately - but we don't know what you've done before this.
Sample:
using System;
using System.Globalization;
class Test
{
static void Main()
{
DateTime epoch = new DateTime(1, 1, 1, new PersianCalendar());
// Prints 0622/03/21 00:00:00
Console.WriteLine(FormatDateTimeAsGregorian(epoch));
}
public static string FormatDateTimeAsGregorian(DateTime input)
{
return input.ToString("yyyy'/'MM'/'dd' 'HH':'mm':'ss",
CultureInfo.InvariantCulture);
}
}
Now if you're not specifying the calendar when you create the DateTime, then you're not really creating a Persian date at all.
If you want dates that keep track of their calendar system, you can use my Noda Time project, which now supports the Persian calendar:
// In Noda Time 2.0 you'd just use CalendarSystem.Persian
var persianDate = new LocalDate(1390, 7, 18, CalendarSystem.GetPersianCalendar());
var gregorianDate = persianDate.WithCalendar(CalendarSystem.Iso);
