SQL Server 2019

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select 
    StartDate, EndDate
into mytable
from (
values
      (cast(getdate() as date),cast(getdate() as date)) -- both same date
    -- original
    ,('2009-04-16','2009-05-15') -- > 0 full month
    ,('2009-04-16','2009-05-16') -- > 1 full month
    ,('2009-04-16','2009-06-16') -- > 2 full months

    -- '1/31/2018' and endDate of '3/1/2018', I get a 0 – Eugene
    , ('2018-01-31','2018-03-01')

    -- some extras mentioned in comments, both of these should return 0 (in my opinion)
    ,('2009-01-31','2009-02-28')
    ,('2012-12-31','2013-02-28')

    ,('2022-05-15','2022-04-16') -- > 0 full month
    ,('2022-05-16','2022-04-16') -- > 1 full month
    ,('2021-06-16','2022-04-16') -- > 10 full months

    ) d (StartDate, EndDate)
10 rows affected

CREATE FUNCTION FullMonthsSeparation 
(
    @DateA DATETIME,
    @DateB DATETIME
)
RETURNS INT
AS
BEGIN
    DECLARE @Result INT

    DECLARE @DateX DATETIME
    DECLARE @DateY DATETIME

    IF(@DateA < @DateB)
    BEGIN
        SET @DateX = @DateA
        SET @DateY = @DateB
    END
    ELSE
    BEGIN
        SET @DateX = @DateB
        SET @DateY = @DateA
    END

    SET @Result = (
                    SELECT 
                    CASE 
                        WHEN DATEPART(DAY, @DateX) > DATEPART(DAY, @DateY)
                        THEN DATEDIFF(MONTH, @DateX, @DateY) - 1
                        ELSE DATEDIFF(MONTH, @DateX, @DateY)
                    END
                    )

    RETURN @Result
END
select
      StartDate
    , EndDate
    , CASE WHEN StartDate <= EndDate THEN
          (DATEPART(YEAR, EndDate) - DATEPART(YEAR, StartDate)) * 12 
        + (DATEPART(MONTH, EndDate) - DATEPART(MONTH, StartDate))
        - CASE WHEN DATEPART(DAY,EndDate) < DATEPART(DAY,StartDate) THEN 1 ELSE 0 END
      ELSE
          (DATEPART(YEAR, StartDate) - DATEPART(YEAR, EndDate)) * 12 
        + (DATEPART(MONTH, StartDate) - DATEPART(MONTH, EndDate))
        - CASE WHEN DATEPART(DAY,StartDate) < DATEPART(DAY,EndDate) THEN 1 ELSE 0 END
      END                                          AS FullMnthsBtwn
from mytable
order by 1


StartDate
EndDate
FullMnthsBtwn


2009-01-31
2009-02-28
0

2009-04-16
2009-05-15
0

2009-04-16
2009-05-16
1

2009-04-16
2009-06-16
2

2012-12-31
2013-02-28
1

2018-01-31
2018-03-01
1

2021-06-16
2022-04-16
10

2022-05-15
2022-04-16
0

2022-05-16
2022-04-16
1

2024-12-18
2024-12-18
0


select
      StartDate
    , EndDate
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
    , dbo.FullMonthsSeparation(EndDate, StartDate) FullMnthsBtwn_2
from mytable
order by 1


StartDate
EndDate
FullMnthsBtwn_1
FullMnthsBtwn_2


2009-01-31
2009-02-28
0
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
1
1

2018-01-31
2018-03-01
1
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
0
0

2022-05-16
2022-04-16
1
1

2024-12-18
2024-12-18
0
0


select
      StartDate
    , EndDate
    , DATEDIFF(MONTH, DATEADD(DAY,-DAY(StartDate)+1,StartDate),DATEADD(DAY,-DAY(StartDate)+1,EndDate))
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
1
1

2018-01-31
2018-03-01
0
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
-1
0

2022-05-16
2022-04-16
-1
1

2024-12-18
2024-12-18
0
0


select
      StartDate
    , EndDate
, CASE WHEN DATEPART(DAY, StartDate) > DATEPART(DAY, EndDate)   
       THEN DATEDIFF(MONTH, StartDate,  EndDate) - 1
       ELSE DATEDIFF(MONTH, StartDate,  EndDate) 
 END
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1 


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
1
1

2018-01-31
2018-03-01
1
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
-1
0

2022-05-16
2022-04-16
-1
1

2024-12-18
2024-12-18
0
0


select
      StartDate
    , EndDate
, CAST(DATEDIFF(MONTH, StartDate, EndDate) AS float) -
  (DATEPART(dd,StartDate) - 1.0) / DATEDIFF(DAY, StartDate, DATEADD(MONTH, 1, StartDate)) +
  (DATEPART(dd,EndDate)*1.0 ) / DATEDIFF(DAY, EndDate, DATEADD(MONTH, 1, EndDate))
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1   
  


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0.928571428572
0

2009-04-16
2009-05-15
0.983870967741
0

2009-04-16
2009-05-16
1.016129032258
1

2009-04-16
2009-06-16
2.033333333333
2

2012-12-31
2013-02-28
2.032258064517
1

2018-01-31
2018-03-01
0.960829493088
1

2021-06-16
2022-04-16
10.033333333333
10

2022-05-15
2022-04-16
-0.918279569892
0

2022-05-16
2022-04-16
-0.950537634408
1

2024-12-18
2024-12-18
0.032258064516
0


select
      StartDate
    , EndDate
    , CASE
        WHEN DATEADD(Month, DATEDIFF(Month, StartDate, EndDate) +1, StartDate) <= EndDate Then DATEDIFF(Month, StartDate, EndDate) +1
        WHEN DATEADD(Month, DATEDIFF(Month, StartDate, EndDate) , StartDate) <= EndDate Then DATEDIFF(Month, StartDate, EndDate) 
        WHEN DATEADD(Month, DATEDIFF(Month, StartDate, EndDate) -1, StartDate) <= EndDate Then DATEDIFF(Month, StartDate, EndDate) -1
      END as [Actual Months Apart]
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
Actual Months Apart
FullMnthsBtwn_1


2009-01-31
2009-02-28
1
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
2
1

2018-01-31
2018-03-01
1
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
-1
0

2022-05-16
2022-04-16
-1
1

2024-12-18
2024-12-18
0
0


select
      StartDate
    , EndDate
    , case 
        when format(StartDate,'yyyy-MM') != format(EndDate,'yyyy-MM')
         AND DATEPART(DAY,StartDate) > DATEPART(DAY,EndDate)
         AND DATEPART(DAY,EndDate) = DATEPART(DAY,EOMONTH(EndDate))
         then 0
        when format(StartDate,'yyyy-MM') != format(EndDate,'yyyy-MM')
         AND DATEPART(DAY,StartDate) > DATEPART(DAY,EndDate)
         then -1 
        else 0 
      end
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0
0

2009-04-16
2009-05-15
-1
0

2009-04-16
2009-05-16
0
1

2009-04-16
2009-06-16
0
2

2012-12-31
2013-02-28
0
1

2018-01-31
2018-03-01
-1
1

2021-06-16
2022-04-16
0
10

2022-05-15
2022-04-16
0
0

2022-05-16
2022-04-16
0
1

2024-12-18
2024-12-18
0
0


CREATE FUNCTION ufFullMonthDif (@dStart DATE, @dEnd DATE)
RETURNS INT
AS
BEGIN
    DECLARE @dif INT,
            @dEnd2 DATE
    SET @dif = DATEDIFF(MONTH, @dStart, @dEnd)
    SET @dEnd2 = DATEADD (MONTH, @dif, @dStart)
    IF @dEnd2 > @dEnd
        SET @dif = @dif - 1
    RETURN @dif
END
select
      StartDate
    , EndDate
    , dbo.ufFullMonthDif(StartDate, EndDate)
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
1
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
2
1

2018-01-31
2018-03-01
1
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
-1
0

2022-05-16
2022-04-16
-1
1

2024-12-18
2024-12-18
0
0


select
      StartDate
    , EndDate
    , DATEDIFF(MONTH, DATEADD(DAY,-DAY(StartDate)+1,StartDate),DATEADD(DAY,-DAY(StartDate)+1,EndDate))
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
1
1

2018-01-31
2018-03-01
0
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
-1
0

2022-05-16
2022-04-16
-1
1

2024-12-18
2024-12-18
0
0


select
      StartDate
    , EndDate
    , CASE 
        WHEN DATEPART(DAY, StartDate) = 1 
        and DATEPART(DAY, EndDate) = DATEPART(DAY, eomonth(EndDate))
            THEN DATEDIFF(MONTH, StartDate, EndDate) + 1
        WHEN DATEPART(DAY, StartDate) > DATEPART(DAY, EndDate) 
        and DATEPART(DAY, EndDate) != DATEPART(DAY, eomonth(EndDate))
            THEN DATEDIFF(MONTH, StartDate, EndDate) - 1
            ELSE DATEDIFF(MONTH, StartDate, EndDate)
      END
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1      


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
1
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
2
1

2018-01-31
2018-03-01
1
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
-1
0

2022-05-16
2022-04-16
-1
1

2024-12-18
2024-12-18
0
0


CREATE FUNCTION MonthsAndDaysBetween (@fromDt date, @toDt date)
RETURNS decimal(10,2)
AS
BEGIN
    DECLARE @d1 date, @d2 date, @numM int, @numD int, @trc varchar(10);

    IF(@fromDt < @toDt)
    BEGIN
        SET @d1 = @fromDt;
        SET @d2 = @toDt;
    END
    ELSE
    BEGIN
        SET @d1 = @toDt;
        SET @d2 = @fromDt;
    END

    IF DAY(@d1)>DAY(@d2)
        SET @numM = year(@d2)*12+month(@d2)-year(@d1)*12-month(@d1)-1;
    ELSE
        SET @numM = year(@d2)*12+month(@d2)-year(@d1)*12-month(@d1);

    IF YEAR(@d1) < YEAR(@d2) OR (YEAR(@d1) = YEAR(@d2) AND MONTH(@d1) < MONTH(@d2))
    BEGIN
        IF DAY(@d2) < DAY(@d1)
            SET @numD = DAY(@d2) + DAY(EOMONTH(DATEADD(month,-1,@d2))) - DAY(@d1);
        ELSE
            SET @numD = DAY(@d2)-DAY(@d1);
    END
    ELSE
        SET @numD = DAY(@d2)-DAY(@d1);

    RETURN @numM + @numD / 100.0;
END
select
      StartDate
    , EndDate
    , dbo.MonthsAndDaysBetween(StartDate, EndDate)
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0.28
0

2009-04-16
2009-05-15
0.29
0

2009-04-16
2009-05-16
1.00
1

2009-04-16
2009-06-16
2.00
2

2012-12-31
2013-02-28
1.28
1

2018-01-31
2018-03-01
0.98
1

2021-06-16
2022-04-16
10.00
10

2022-05-15
2022-04-16
0.29
0

2022-05-16
2022-04-16
1.00
1

2024-12-18
2024-12-18
0.00
0


select
      StartDate
    , EndDate
    , (DATEPART(YEAR, EndDate) - DATEPART(YEAR, StartDate)) * 12 
    + (DATEPART(MONTH, EndDate) - DATEPART(MONTH, StartDate))
    - CASE WHEN DATEPART(DAY,EndDate) < DATEPART(DAY,StartDate) THEN 1 ELSE 0 END
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
1
1

2018-01-31
2018-03-01
1
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
-1
0

2022-05-16
2022-04-16
-1
1

2024-12-18
2024-12-18
0
0


select
      StartDate
    , EndDate
    , CASE WHEN StartDate <= EndDate THEN
          (DATEPART(YEAR, EndDate) - DATEPART(YEAR, StartDate)) * 12 
        + (DATEPART(MONTH, EndDate) - DATEPART(MONTH, StartDate))
        - CASE WHEN DATEPART(DAY,EndDate) < DATEPART(DAY,StartDate) THEN 1 ELSE 0 END
      ELSE
          (DATEPART(YEAR, StartDate) - DATEPART(YEAR, EndDate)) * 12 
        + (DATEPART(MONTH, StartDate) - DATEPART(MONTH, EndDate))
        - CASE WHEN DATEPART(DAY,StartDate) < DATEPART(DAY,EndDate) THEN 1 ELSE 0 END
      END
    , dbo.FullMonthsSeparation(StartDate, EndDate) FullMnthsBtwn_1
from mytable
order by 1


StartDate
EndDate
(No column name)
FullMnthsBtwn_1


2009-01-31
2009-02-28
0
0

2009-04-16
2009-05-15
0
0

2009-04-16
2009-05-16
1
1

2009-04-16
2009-06-16
2
2

2012-12-31
2013-02-28
1
1

2018-01-31
2018-03-01
1
1

2021-06-16
2022-04-16
10
10

2022-05-15
2022-04-16
0
0

2022-05-16
2022-04-16
1
1

2024-12-18
2024-12-18
0
0
StartDate	EndDate	FullMnthsBtwn
2009-01-31	2009-02-28	0
2009-04-16	2009-05-15	0
2009-04-16	2009-05-16	1
2009-04-16	2009-06-16	2
2012-12-31	2013-02-28	1
2018-01-31	2018-03-01	1
2021-06-16	2022-04-16	10
2022-05-15	2022-04-16	0
2022-05-16	2022-04-16	1
2024-12-18	2024-12-18	0
StartDate	EndDate	(No column name)	FullMnthsBtwn_1
2009-01-31	2009-02-28	0.928571428572	0
2009-04-16	2009-05-15	0.983870967741	0
2009-04-16	2009-05-16	1.016129032258	1
2009-04-16	2009-06-16	2.033333333333	2
2012-12-31	2013-02-28	2.032258064517	1
2018-01-31	2018-03-01	0.960829493088	1
2021-06-16	2022-04-16	10.033333333333	10
2022-05-15	2022-04-16	-0.918279569892	0
2022-05-16	2022-04-16	-0.950537634408	1
2024-12-18	2024-12-18	0.032258064516	0
StartDate	EndDate	(No column name)	FullMnthsBtwn_1
2009-01-31	2009-02-28	0.28	0
2009-04-16	2009-05-15	0.29	0
2009-04-16	2009-05-16	1.00	1
2009-04-16	2009-06-16	2.00	2
2012-12-31	2013-02-28	1.28	1
2018-01-31	2018-03-01	0.98	1
2021-06-16	2022-04-16	10.00	10
2022-05-15	2022-04-16	0.29	0
2022-05-16	2022-04-16	1.00	1
2024-12-18	2024-12-18	0.00	0