I'm in the process of developing a complex scheduler that will allow users to add/subtract "6 Months", "2 Weeks", etc.  For the most part that's pretty straightforward, but it does complicated with months because of their varying length.  Test code is in the reply.

If making any modifications be careful because sometimes you are adding a negative, and the tendency can be to think "I need to take the difference between Month and AmountToAdd because the result will be earlier than the original date."

// ======================================================================
// ADD TO DATE
// Returns Old date plus X days, weeks, months or years as Date
// AMOUNT TO ADD CAN BE NEGATIVE
// ======================================================================
Function AddToDate(OldDate as Date, AddWhat as String, AmountToAdd as Int) as Date
     var NewDate as Date = OldDate
     var YearNum as Int = @Year(OldDate)
     var MonthNum as Int = @Month(OldDate)

     AddWhat = @Replace(ToUpper(AddWhat), "S","") // Convert to uppercase, remove final S if any

     If AmountToAdd = 0 then // Do nothing -- date will be returned unchanged
     Else If AddWhat = "DAY" then NewDate = OldDate + AmountToAdd
     Else If AddWhat = "WEEK" then NewDate = OldDate + (7 * AmountToAdd)
     Else If AddWhat = "YEAR" then NewDate = @ToDate(@str(YearNum + AmountToAdd) + @Right(OldDate, 6))
     Else If AddWhat <> "MONTH" then // Do nothing -- date will be returned unchanged

     Else If AmountToAdd > 0 then      // ADD months
     {
           MonthNum = MonthNum + AmountToAdd      // E.g. 11/23/2003 + 27 = 38/23/2003, Months = 38
           YearNum = YearNum + (MonthNum /12)
           MonthNum = @Mod(MonthNum, 12)
           If MonthNum = 0 Then MonthNum = 12 // December mod 12 = Month 0
           NewDate = @ToDate(@Str(YearNum) + "/" + @Right("0" + @Str(MonthNum),2) + @Right(OldDate, 3))
     }
     Else  // SUBTRACT months
     {
           /* Vars:
           MonthNum = current month
           AmountToAdd = Amount to SUBTRACT
           YearNum = current year      
           */
           // Example: subtract 22 months from 1998/02/18
           YearNum = YearNum + @Int(AmountToAdd / 12)            // How many years to subtract = YearNum is now 1997
           AmountToAdd = @Mod(AmountToAdd, 12)            // How many months (less than yr) to subtract = 10.
           // Year is now correct but months not changed yet
           If AmountToAdd = 0      // 12 mos, 24, etc. -- Do nothing
           Else If (AmountToAdd + MonthNum) < 1 Then      // e.g., subtract 2 to 11 months from February
           {
                       YearNum = YearNum - 1                  // Do subtraction "borrowing" of 12 months.  YearNum is now 1996
                       MonthNum = MonthNum + 12 + AmountToAdd  // ADD MINUS 10 months to 14 months. Month is now 4 = April
           }
           Else MonthNum = MonthNum + AmountToAdd

           NewDate = @ToDate(@Str(YearNum) + "/" + @Right("0" + @Str(MonthNum),2) + @Right(OldDate, 3))
     }
     Return(NewDate)
End Function // AddToDate

A caution regarding the routine, that I didn't originally notice --

Scenario: You receive a communication to "Meet me 1 month from today."  If you don't, someone will die (for whatever reason).  You can't communicate with the sender and (again, for whatever reason) you can only go on one day.

The communication is dated January, March, August or October 31. (Each followed by a month with less than 31 days.)

Which date is "1 month from" that date?

Related issue: Which date is "1 month before" March 31, July 31, etc?

I'll probably set up a poll to see if there a consensus.