SQL Server 2017

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CREATE FUNCTION [dbo].[tvf-Date-Elapsed] (@D1 DateTime,@D2 DateTime)
Returns Table
Return (
    with cteBN(N)   as (Select 1 From (Values(1),(1),(1),(1),(1),(1),(1),(1),(1),(1)) N(N)),
         cteRN(R)   as (Select Row_Number() Over (Order By (Select NULL))-1 From cteBN a,cteBN b,cteBN c),
         cteYY(N,D) as (Select Max(R),Max(DateAdd(YY,R,@D1))From cteRN R Where DateAdd(YY,R,@D1)<=@D2),
         cteMM(N,D) as (Select Max(R),Max(DateAdd(MM,R,D))  From (Select Top 12 R From cteRN Order By 1) R, cteYY P Where DateAdd(MM,R,D)<=@D2),
         cteDD(N,D) as (Select Max(R),Max(DateAdd(DD,R,D))  From (Select Top 31 R From cteRN Order By 1) R, cteMM P Where DateAdd(DD,R,D)<=@D2),
         cteHH(N,D) as (Select Max(R),Max(DateAdd(HH,R,D))  From (Select Top 24 R From cteRN Order By 1) R, cteDD P Where DateAdd(HH,R,D)<=@D2),
         cteMI(N,D) as (Select Max(R),Max(DateAdd(MI,R,D))  From (Select Top 60 R From cteRN Order By 1) R, cteHH P Where DateAdd(MI,R,D)<=@D2),
         cteSS(N,D) as (Select Max(R),Max(DateAdd(SS,R,D))  From (Select Top 60 R From cteRN Order By 1) R, cteMI P Where DateAdd(SS,R,D)<=@D2)

    Select [Years]   = cteYY.N
          ,[Months]  = cteMM.N
          ,[Days]    = cteDD.N
          ,[Hours]   = cteHH.N
          ,[Minutes] = cteMI.N
          ,[Seconds] = cteSS.N
          --,[Elapsed] = Format(cteYY.N,'0000')+':'+Format(cteMM.N,'00')+':'+Format(cteDD.N,'00')+' '+Format(cteHH.N,'00')+':'+Format(cteMI.N,'00')+':'+Format(cteSS.N,'00')
     From  cteYY,cteMM,cteDD,cteHH,cteMI,cteSS
)
CREATE FUNCTION [dbo].[tvf_interval_parts] (@start DateTime, @cease DateTime)
Returns Table
RETURN
  SELECT
    *
  FROM
  (
    SELECT
      months / 12   AS years,
      months % 12   AS months
    FROM
    (
      SELECT
        CASE WHEN DATEADD(MONTH, months, @start) > @cease                           THEN months - 1
             WHEN DATEADD(MONTH, months, @start) = DATEADD(MONTH, months, @start-1) THEN months - 1
                                                                                    ELSE months     END   AS months
      FROM
      (
        SELECT DATEDIFF(MONTH, @start, @cease) AS months
      )
        provisional
    )
      adjusted
  )
    interim
  CROSS APPLY
  (
      SELECT
        milliseconds                         / (24 * 60 * 60 * 1000)  AS days,
        milliseconds % (24 * 60 * 60 * 1000) / (     60 * 60 * 1000)  AS hours,
        milliseconds % (     60 * 60 * 1000) / (          60 * 1000)  AS minutes,
        milliseconds % (          60 * 1000) / (               1000)  AS seconds,
        milliseconds % (               1000)                          AS milliseconds
      FROM
      (
        SELECT DATEDIFF_BIG(millisecond, DATEADD(MONTH, 12 * years + months, @start), @cease) AS milliseconds
      )
        provisional
  )
    remainder
CREATE FUNCTION [dbo].[tvf_date_add_interval_parts] (@base DateTime, @years INT, @months INT, @days INT, @hours INT, @minutes INT, @seconds INT, @milliseconds INT)
Returns Table
RETURN
  SELECT
    DATEADD(MILLISECOND, @milliseconds,
      DATEADD(SECOND, @seconds,
        DATEADD(MINUTE, @minutes,
          DATEADD(HOUR, @hours,
            DATEADD(DAY, @days,
              DATEADD(MONTH, @months,
                DATEADD(YEAR, @years,
                  @base
                )))))))  AS result
WITH
  base AS
(
            SELECT CAST('2015-01-27' AS DATETIME)  AS start, CAST('2016-02-29' AS DATETIME)  AS cease
  UNION ALL SELECT CAST('2015-01-28' AS DATETIME)  AS start, CAST('2016-02-29' AS DATETIME)  AS cease
  UNION ALL SELECT CAST('2015-01-29' AS DATETIME)  AS start, CAST('2016-02-29' AS DATETIME)  AS cease
  UNION ALL SELECT CAST('2015-01-30' AS DATETIME)  AS start, CAST('2016-02-29' AS DATETIME)  AS cease
  UNION ALL SELECT CAST('2015-01-31' AS DATETIME)  AS start, CAST('2016-02-29' AS DATETIME)  AS cease
  UNION ALL SELECT CAST('2015-02-01' AS DATETIME)  AS start, CAST('2016-02-29' AS DATETIME)  AS cease
  UNION ALL SELECT CAST('2016-02-29' AS DATETIME)  AS start, CAST('2017-03-28' AS DATETIME)  AS cease
)
SELECT
  *
FROM
  base
CROSS APPLY
  [dbo].[tvf_interval_parts](base.start, base.cease)   AS v1
CROSS APPLY
  [dbo].[tvf_date_add_interval_parts](base.start, v1.years, v1.months, v1.days, v1.hours, v1.minutes, v1.seconds, v1.milliseconds)   AS check1
CROSS APPLY
  [dbo].[tvf-Date-Elapsed](base.start, base.cease)   AS v2
CROSS APPLY
  [dbo].[tvf_date_add_interval_parts](base.start, v2.years, v2.months, v2.days, v2.hours, v2.minutes, v2.seconds, 0              )   AS check2


start
cease
years
months
days
hours
minutes
seconds
milliseconds
result
Years
Months
Days
Hours
Minutes
Seconds
result


2015-01-27 00:00:00.000
2016-02-29 00:00:00.000
1
1
2
0
0
0
0
2016-02-29 00:00:00.000
1
1
2
0
0
0
2016-02-29 00:00:00.000

2015-01-28 00:00:00.000
2016-02-29 00:00:00.000
1
1
1
0
0
0
0
2016-02-29 00:00:00.000
1
1
1
0
0
0
2016-02-29 00:00:00.000

2015-01-29 00:00:00.000
2016-02-29 00:00:00.000
1
1
0
0
0
0
0
2016-02-29 00:00:00.000
1
1
0
0
0
0
2016-02-29 00:00:00.000

2015-01-30 00:00:00.000
2016-02-29 00:00:00.000
1
0
30
0
0
0
0
2016-02-29 00:00:00.000
1
1
0
0
0
0
2016-02-29 00:00:00.000

2015-01-31 00:00:00.000
2016-02-29 00:00:00.000
1
0
29
0
0
0
0
2016-02-29 00:00:00.000
1
1
0
0
0
0
2016-02-29 00:00:00.000

2015-02-01 00:00:00.000
2016-02-29 00:00:00.000
1
0
28
0
0
0
0
2016-02-29 00:00:00.000
1
0
28
0
0
0
2016-02-29 00:00:00.000

2016-02-29 00:00:00.000
2017-03-28 00:00:00.000
1
0
28
0
0
0
0
2017-03-28 00:00:00.000
1
1
0
0
0
0
2017-03-28 00:00:00.000


WITH
  base AS
(
            SELECT CAST('2015-01-29 01:01:01.001' AS DATETIME)  AS start, CAST('2016-02-29 02:02:02.002' AS DATETIME)  AS cease
  UNION ALL SELECT CAST('2015-01-29 02:02:02.002' AS DATETIME)  AS start, CAST('2016-02-29 01:01:01.001' AS DATETIME)  AS cease
)
SELECT
  *
FROM
  base
CROSS APPLY
  [dbo].[tvf_interval_parts](base.start, base.cease)   AS v1
CROSS APPLY
  [dbo].[tvf_date_add_interval_parts](base.start, v1.years, v1.months, v1.days, v1.hours, v1.minutes, v1.seconds, v1.milliseconds)   AS check1
CROSS APPLY
  [dbo].[tvf-Date-Elapsed](base.start, base.cease)   AS v2
CROSS APPLY
  [dbo].[tvf_date_add_interval_parts](base.start, v2.years, v2.months, v2.days, v2.hours, v2.minutes, v2.seconds, 0              )   AS check2


start
cease
years
months
days
hours
minutes
seconds
milliseconds
result
Years
Months
Days
Hours
Minutes
Seconds
result


2015-01-29 01:01:01.000
2016-02-29 02:02:02.003
1
1
0
1
1
1
3
2016-02-29 02:02:02.003
1
1
0
1
1
1
2016-02-29 02:02:02.000

2015-01-29 02:02:02.003
2016-02-29 01:01:01.000
1
0
30
22
58
58
997
2016-02-29 01:01:01.000
1
0
30
22
58
58
2016-02-29 01:01:00.003

start	cease	years	months	days	result	Years	Months	Days	result
2015-01-27 00:00:00.000	2016-02-29 00:00:00.000	1	1	2	2016-02-29 00:00:00.000	1	1	2	2016-02-29 00:00:00.000
2015-01-28 00:00:00.000	2016-02-29 00:00:00.000	1	1	1	2016-02-29 00:00:00.000	1	1	1	2016-02-29 00:00:00.000
2015-01-29 00:00:00.000	2016-02-29 00:00:00.000	1	1	0	2016-02-29 00:00:00.000	1	1	0	2016-02-29 00:00:00.000
2015-01-30 00:00:00.000	2016-02-29 00:00:00.000	1	0	30	2016-02-29 00:00:00.000	1	1	0	2016-02-29 00:00:00.000
2015-01-31 00:00:00.000	2016-02-29 00:00:00.000	1	0	29	2016-02-29 00:00:00.000	1	1	0	2016-02-29 00:00:00.000
2015-02-01 00:00:00.000	2016-02-29 00:00:00.000	1	0	28	2016-02-29 00:00:00.000	1	0	28	2016-02-29 00:00:00.000
2016-02-29 00:00:00.000	2017-03-28 00:00:00.000	1	0	28	2017-03-28 00:00:00.000	1	1	0	2017-03-28 00:00:00.000

start	cease	years	months	days	hours	minutes	seconds	milliseconds	result	Years	Months	Days	Hours	Minutes	Seconds	result
2015-01-29 01:01:01.000	2016-02-29 02:02:02.003	1	1	0	1	1	1	3	2016-02-29 02:02:02.003	1	1	0	1	1	1	2016-02-29 02:02:02.000
2015-01-29 02:02:02.003	2016-02-29 01:01:01.000	1	0	30	22	58	58	997	2016-02-29 01:01:01.000	1	0	30	22	58	58	2016-02-29 01:01:00.003