c# - What is the exact Excel Days360 algorithm? -

i'm porting calculations excel c# use days360 function (the default/us method). using wikipedia page guide, came code:

    public static int days360(datetime a, datetime b)     {         var daya = a.day;         var dayb = b.day;          if (islastdayoffebruary(a) && islastdayoffebruary(b))             dayb = 30;          if (daya == 31 || islastdayoffebruary(a))             daya = 30;          if (daya == 30 && dayb == 31)             dayb = 30;          return ((b.year - a.year) * 12 + b.month - a.month) * 30 + dayb - daya;     }      private static bool islastdayoffebruary(datetime date)     {         if (date.month != 2)             return false;          int lastday = datetime.daysinmonth(date.year, 2);         return date.day == lastday;     } 

i tested (small) range of inputs , results agree excel's native function except if use 2015-02-28 both , b. code returns 0 , excel -2.

my result seems more reasonable @ point, i'd prefer calculate exact same result excel. there might other inputs disagree don't want make special case date.

does know exact algorithm excel uses?

edit: there glaring bug in original code posted unrelated question. had fixed 1 copied wrong file when posting question.

according this wikipedia article microsoft excel days360 function equivalent 30/360 bma/psa. exact results ms excel need implement bma/psa method. have implemented method.

private double days360(datetime startdate, datetime enddate) {     int startday = startdate.day;     int startmonth = startdate.month;     int startyear = startdate.year;     int endday = enddate.day;     int endmonth = enddate.month;     int endyear = enddate.year;      if (startday == 31 || islastdayoffebruary(startdate))     {         startday = 30;     }      if (startday == 30 && endday == 31)     {         endday = 30;     }      return ((endyear - startyear) * 360) + ((endmonth - startmonth) * 30) + (endday - startday); }  private bool islastdayoffebruary(datetime date) {     return date.month == 2 && date.day == datetime.daysinmonth(date.year, date.month); }