SQL Date/Interval Casting Puzzle

One of my non-Informix developers is writing a report and needs to find records in an Informix table with begin_date X number of months, or a multiple of X number of months, prior to a static date. In his own words: I Just need to fill in the Infomix syntax of it. Input: ParameterMonth ParameterYear ParameterInterval DB Field : BeginDate Criteria for selecting a record: BeginDate must be a multiple of ParameterInterval months in the past. ie. if the interval is 5 then the begin date must be 5 months ago..ten months ago...15 months ago............. to be included What I am looking to do is something along these lines...forgive the pseudocode 1. Create a date from the input year and month (Call it NewDate for this example) 2.Use some kind of a datediff function to return the # of months between the potentially selected record's begin date and NewDate 3. Divide the integer returned from the datediff function by the ParameterInterval 4. If the Modulus(Remainder) is 0 then the record gets selected the pseudo code would look something like this Select * from tables where Modulus (datediff (month,date(parameteryear,Parametermonth,01),BeginDate)/ParameterInterval) =0 Any takers on turning this into real Informix SQL? I haven't been able to get the SQL to work because I haven't been able to figure how to cast a difference between two dates into an integer representing months in order to take a modulus of the difference. This is the rudimentary SQL I came up with: select date("01/01/2000") const_dt, begin_date original_dt, extend(date("01/01/2000") - 5 units month, year to month) constdt_varduratn, extend(begin_date, year to month) extended_orig_dt from <tablename> where extend(begin_date, year to month) = extend(date("01/01/2000") - 5 units month, year to month); It'll do it for the first batch 5 months back, but, obviously, not 10 or 15 or 20, etc. Is there a solution to this puzzle?