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

The explanation by David Yaw is spot on, but calculation of the cumulative number of days of the year for the months prior to the given month is anti-intuitive. If you prefer an array of integers to make the algorithm more clear then this will do:

    /*
     * convert magic numbers created by:
     *    (153*month - 457)/5) 
     * into an explicit array of integers
     */
    int[] CumulativeDays = new int[]
    {
        -92   // Month = 0  (Should not be accessed by algorithm)
      , -61   // Month = 1  (Should not be accessed by algorithm)
      , -31   // Month = 2  (Should not be accessed by algorithm)
      ,   0   // Month = 3  (March)
      ,  31   // Month = 4  (April)
      ,  61   // Month = 5  (May)
      ,  92   // Month = 6  (June)
      , 122   // Month = 7  (July)
      , 153   // Month = 8  (August)
      , 184   // Month = 9  (September)
      , 214   // Month = 10 (October)
      , 245   // Month = 11 (November)
      , 275   // Month = 12 (December)
      , 306   // Month = 13 (January, next year)
      , 337   // Month = 14 (February, next year)
    };

and the first thre lines of the calculation then become:

  int julianDay = day
                  + CumulativeDays[month]
                  + 365*year
                  + (year/4)

The expression

(153*month - 457)/5)

though produces the exact same sequence same integers as the array above for values in the range: 3 to 14; inclusive and does so with no storage requirements. The lack of storage requirements is only virtue in calculating the cumulative number of days in such and obfuscated way.