private DateTime GetDate(DateTime inputDate, string differenceType, bool add, int difference)
{
DateTime result = inputDate;

if (!add)
{
//Substraction
difference = -1 * difference;
}

switch (differenceType.ToUpper())
{
case "DAYS":
result = new DateTime(inputDate.Year, inputDate.Month, inputDate.Day).AddDays(difference);
break;

case "MONTHS":
result = new DateTime(inputDate.Year, inputDate.Month, inputDate.Day).AddMonths(difference);
break;

case "YEARS":
result = new DateTime(inputDate.Year, inputDate.Month, inputDate.Day).AddYears(difference);
break;
}
return result;
}

Calling function:
DateTime result = GetDate(DateTime.Now, "DAYS", false, 15);