Calculate days remaining to a birthday?

current code is for age calculate

 int[] monthDay = new int[12] { 31, -1, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
            // contain birth date
            DateTime birthdate = this.birthday.Date.DateTime;
            // cntain current date
            DateTime currentdate = this.calculateday.Date.DateTime;
            int day , year , month;
          
            
            DateTime d1 = birthdate ;
            DateTime d2 = currentdate;
            
            int increment;
            
            if(d1>d2)
            {
                birthdate = d2;
                currentdate = d1;
            }
            else
            {
                birthdate = d1;
                currentdate = d2;
            }
            // day calculation
            increment = 0;
            if(birthdate.Day > currentdate.Day)
            {
                increment = monthDay[birthdate.Month - 1];
            }
             /// if it is february month
            /// if it's to day is less then from day
            if (increment == -1)
            {
                if(DateTime.IsLeapYear(birthdate.Year))
                {
                    // leap year february contain 29 days
                    increment = 29;
                }
                else
                {
                    increment = 28;
                }
            }
             if (increment != 0)
            {
                day = (currentdate.Day + increment) - birthdate.Day;
                increment = 1;
            }
            else
            {
                day = currentdate.Day - birthdate.Day;
            }

            ///month calculation
            if ((birthdate.Month + increment) > currentdate.Month)
            {
                month = (currentdate.Month + 12) - (birthdate.Month + increment);
                increment = 1;
            }
            else
            {
                month = (currentdate.Month) - (birthdate.Month + increment);
                increment = 0;
            }
            /// year calculation
            year = currentdate.Year - (birthdate.Year + increment);