Convert DateTime to Julian Date in C# (ToOADate Safe?)

Question

I need to convert from a standard Gregorian date to a Julian day number.

I\'ve seen nothing documented in C# to do this directly,

Answer 1

My code for modified Julian Date uses the same algorithm but a different magic number on the end so that the resulting value matches the Modified Julian Date shown on Wikipedia. I have been using this same algorithm for at least 10 years as the key for daily time series (originally in Java).

public static int IntegerDate(DateTime date)
    {
        int Month = date.Month;
        int Day = date.Day;
        int Year = date.Year;

        if (Month < 3)
        {
            Month = Month + 12;
            Year = Year - 1;
        }
        //modified Julian Date
        return Day + (153 * Month - 457) / 5 + 365 * Year + (Year / 4) - (Year / 100) + (Year / 400) - 678882;
    }

The reverse calculation has more magic numbers for your amusement:

public static DateTime FromDateInteger(int mjd)
    {
        long a = mjd + 2468570;
        long b = (long)((4 * a) / 146097);
        a = a - ((long)((146097 * b + 3) / 4));
        long c = (long)((4000 * (a + 1) / 1461001));
        a = a - (long)((1461 * c) / 4) + 31;
        long d = (long)((80 * a) / 2447);
        int Day = (int)(a - (long)((2447 * d) / 80));
        a = (long)(d / 11);
        int Month = (int)(d + 2 - 12 * a);
        int Year = (int)(100 * (b - 49) + c + a);
        return new DateTime(Year, Month, Day);
    }