Calculate DayOfCentury for C# DateTime, similar to DayOfYear

DayOfCentury.cs

/// <description>
/// Calculates the total number of days passed sind first of January 2000
/// until a given Date.
/// </description>
public static long DayOfCentury(DateTime dt)
{
	var span = dt.Date - new DateTime(2000, 1, 1);

	return (long)span.TotalDays;
}