DECLARE @MinDate DATE = (SELECT MIN(StartDate) FROM SampleData)
DECLARE @MaxDate DATE = (SELECT MAX(EndDate) FROM SampleData)
;WITH n AS 
(
SELECT 0 AS n
UNION ALL
SELECT n + 1 
FROM n 
WHERE n + 1 <= DATEDIFF(DAY, @MinDate, @MaxDate)
)
SELECT DATEADD(DAY, n, @MinDate) AS Date
INTO #Calendar
FROM n 
OPTION (MAXRECURSION 0)
CREATE TABLE #Result
(
Date DATE,
[Group] VARCHAR(20),
[Location] VARCHAR(100),
[Amount] DECIMAL(10,2),
Counted INT,
CumulativeCount INT,
ResetDate DATE
);
DECLARE @Group VARCHAR(20), @Date DATE, @Location VARCHAR(100), @Amount DECIMAL(10,2);
DECLARE GroupCursor CURSOR FOR 
SELECT DISTINCT [Group], [Location], [Amount] FROM SampleData
OPEN GroupCursor
FETCH NEXT FROM GroupCursor INTO @Group, @Location, @Amount
WHILE @@FETCH_STATUS = 0
BEGIN
DECLARE DateCursor CURSOR FOR 
SELECT Date FROM #Calendar ORDER BY Date
OPEN DateCursor
FETCH NEXT FROM DateCursor INTO @Date
DECLARE @CumulativeCount INT = 0, @ResetDate DATE = NULL
WHILE @@FETCH_STATUS = 0
BEGIN
DECLARE @Counted INT = 
(
SELECT CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END
FROM SampleData
WHERE [Group] = @Group AND @Date BETWEEN StartDate AND EndDate
)
IF @Counted = 1 AND (@CumulativeCount = 0 OR DATEDIFF(DAY, @ResetDate, @Date) >= DATEDIFF(DAY, @ResetDate, DATEADD(YEAR, 1, @ResetDate))) --- Altered check to account for leap years. 
BEGIN
SET @CumulativeCount = 1
SET @ResetDate = @Date  
END
ELSE IF @Counted = 1 AND @CumulativeCount < 100 
SET @CumulativeCount = @CumulativeCount + 1
ELSE IF @Counted = 1 AND @CumulativeCount >= 100
SET @Counted = 0
IF  
( 
SELECT CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END
FROM SampleData
WHERE [Group] = @Group AND [Location] = @Location AND [Amount] = @Amount AND @Date BETWEEN StartDate AND EndDate
) = 1
BEGIN
INSERT INTO #Result (Date, [Group], [Location], [Amount], Counted, CumulativeCount, ResetDate)
VALUES (@Date, @Group, @Location, @Amount, @Counted, @CumulativeCount, @ResetDate)
END
FETCH NEXT FROM DateCursor INTO @Date
END
CLOSE DateCursor
DEALLOCATE DateCursor
FETCH NEXT FROM GroupCursor INTO @Group, @Location, @Amount
END
CLOSE GroupCursor
DEALLOCATE GroupCursor
SELECT * FROM #Result
ORDER BY [Group], Date
SELECT 
[Group],
[Location],
[Amount],
FORMAT(Date, 'yyyyMM') AS YearMonth,
SUM(Counted) AS CountDays
FROM #Result
GROUP BY [Group], [Location], [Amount], FORMAT(Date, 'yyyyMM')
ORDER BY [Group], YearMonth, [Location]

SELECT t.GroupID, t.Location, t.StartDate, t.EndDate, c.Year, c.Month, COUNT(CASE WHEN DATEDIFF(DAY,(DATEADD(YEAR,DATEDIFF(YEAR,t.StartDate,c.Date),t.StartDate)),c.Date) < 100 THEN 1 END) AS CountDays
  FROM @Table t
    INNER JOIN dbo.Calendar(3,1) c
	  ON t.StartDate <= c.Date
	  AND t.EndDate >= c.Date
 GROUP BY t.GroupID, t.Location, t.StartDate, t.EndDate, c.Year, c.Month
 ORDER BY GroupID, Location, c.Year, c.Month

;WITH CalendarCTE AS (
SELECT MIN(StartDate) AS Date, MAX(EndDate) AS mxDate, DATEPART(MONTH,MIN(StartDate)) AS Month, DATEPART(YEAR,MIN(StartDate)) AS Year
FROM @Table
UNION ALL
SELECT DATEADD(DAY,1,Date), mxDate, DATEPART(MONTH,DATEADD(DAY,1,Date)) AS Month, DATEPART(YEAR,DATEADD(DAY,1,Date)) AS Year
  FROM CalendarCTE
 WHERE Date < mxDate
)
SELECT t.GroupID, t.Location, t.StartDate, t.EndDate, c.Year, c.Month, COUNT(CASE WHEN DATEDIFF(DAY,(DATEADD(YEAR,DATEDIFF(YEAR,t.StartDate,c.Date),t.StartDate)),c.Date) < 100 THEN 1 END) AS CountDays
  FROM @Table t
    INNER JOIN CalendarCTE c
	  ON t.StartDate <= c.Date
	  AND t.EndDate >= c.Date
 GROUP BY t.GroupID, t.Location, t.StartDate, t.EndDate, c.Year, c.Month
 ORDER BY GroupID, Location, c.Year, c.Month
   OPTION (MAXRECURSION 0)

with explode as (
select *,
date_bucket(year, 1, Mnth,
min(StartDate) over (partition by [Group])) as ReferenceDate,
case when Mnth <> eomonth(EndDate) then day(Mnth) else 1 end as Counter
from SampleData data cross apply (
select eomonth(dateadd(month, value, StartDate)) as Mnth
from generate_series(0, 99) -- adjust if 100 months isn't enough...
where dateadd(month, value, StartDate) <= EndDate
) months
), accumulate as (
select *,
sum(Counter) over (partition by [Group], ReferenceDate order by Mnth) as CumDays
from explode
)
select [Group], Location, Amount,
convert(char(6), Mnth, 112) as YearMonth,
case when CumDays <= 100 then Counter
when CumDays - Counter < 100 then 100 - (CumDays - Counter)
else 0 end as CountDays
from accumulate
order by [Group], YearMonth, Location;