CREATE     FUNCTION [dbo].[fnGetInitialDate] (
       @Month               int = null,
       @Year         int = null,
       @CurrentDate datetime
)
RETURNS int
AS
BEGIN
       Declare @ReportDate datetime
       Declare @InitialMonth      int    

       if @month IS null
              set @month = Month(Dateadd(m, -1, @CurrentDate))

       if @Year IS null
              set @Year = Year(Dateadd(m, -1, @CurrentDate))

-- NORMALISE Date
/*
       Select
              @Date = StartDateTime,
              @DateDiff = AgeInMonths
       from
              (
                     select top 1
                           StartDateTime,
                           AgeInMonths
                     from
                           dbo.<censored>
                     where
                            StartDateTime is not null
              ) TA
*/

       set @ReportDate      = Cast(@Year AS VARCHAR(4)) + '-' +  Right('00' + Cast(@month AS VARCHAR(2)),2) + '-01'
       set @InitialMonth = datediff(m, @CurrentDate, @ReportDate)

       return @InitialMonth
END

This SQL Server function features:

• A name that doesn’t tell you anything ✔
• Comments indicating non-existent functionality ✔
• Big sections commented out with no explanation ✔
• Using strings for date arithmetic ✔

After much thought I concluded that this code is functionally equivalent to DATEDIFF(MONTH, GETDATE(), @ReportDate), except you can choose a different year or month for the current time.

This function is still at large on a production database server (names have been suppressed to protect the innocent). Developers are recommended not to approach it as it is considered dangerous, and extremely confusing.