Answer by Michael for Difference in months between two dates

This simple static function calculates the fraction of months between two Datetimes, e.g.

• 1.1. to 31.1. = 1.0
• 1.4. to 15.4. = 0.5
• 16.4. to 30.4. = 0.5
• 1.3. to 1.4. = 1 + 1/30

The function assumes that the first date is smaller than the second date. To deal with negative time intervals one can modify the function easily by introducing a sign and a variable swap at the beginning.

public static double GetDeltaMonths(DateTime t0, DateTime t1){     DateTime t = t0;     double months = 0;     while(t<=t1)     {         int daysInMonth = DateTime.DaysInMonth(t.Year, t.Month);         DateTime endOfMonth = new DateTime(t.Year, t.Month, daysInMonth);         int cutDay = endOfMonth <= t1 ? daysInMonth : t1.Day;         months += (cutDay - t.Day + 1) / (double) daysInMonth;         t = new DateTime(t.Year, t.Month, 1).AddMonths(1);     }     return Math.Round(months,2); }