There are not a lot of clear answers on this because you are always assuming things.
This solution calculates between two dates the months between assuming you want to save the day of month for comparison, (meaning that the day of the month is considered in the calculation)
Example, if you have a date of 30 Jan 2012, 29 Feb 2012 will not be a month but 01 March 2013 will.
It's been tested pretty thoroughly, probably will clean it up later as we use it, but here:
private static int TotalMonthDifference(DateTime dtThis, DateTime dtOther){ int intReturn = 0; bool sameMonth = false; if (dtOther.Date < dtThis.Date) //used for an error catch in program, returns -1 intReturn--; int dayOfMonth = dtThis.Day; //captures the month of day for when it adds a month and doesn't have that many days int daysinMonth = 0; //used to caputre how many days are in the month while (dtOther.Date > dtThis.Date) //while Other date is still under the other { dtThis = dtThis.AddMonths(1); //as we loop, we just keep adding a month for testing daysinMonth = DateTime.DaysInMonth(dtThis.Year, dtThis.Month); //grabs the days in the current tested month if (dtThis.Day != dayOfMonth) //Example 30 Jan 2013 will go to 28 Feb when a month is added, so when it goes to march it will be 28th and not 30th { if (daysinMonth < dayOfMonth) // uses day in month max if can't set back to day of month dtThis.AddDays(daysinMonth - dtThis.Day); else dtThis.AddDays(dayOfMonth - dtThis.Day); } if (((dtOther.Year == dtThis.Year) && (dtOther.Month == dtThis.Month))) //If the loop puts it in the same month and year { if (dtOther.Day >= dayOfMonth) //check to see if it is the same day or later to add one to month intReturn++; sameMonth = true; //sets this to cancel out of the normal counting of month } if ((!sameMonth)&&(dtOther.Date > dtThis.Date))//so as long as it didn't reach the same month (or if i started in the same month, one month ahead, add a month) intReturn++; } return intReturn; //return month}