SQL Server 2019

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CREATE TABLE Dates
(
  ID INT IDENTITY,
  StartDate DATE, 
  EndDate DATE
)
INSERT INTO Dates (StartDate, EndDate)
VALUES ('2016-06-09', '2019-12-31'),
  ('2016-06-09', '2019-01-30'),
  ('2019-02-09', '2019-07-30')
3 rows affected

SELECT ID, StartDate, EndDate, 
  (CASE
    WHEN StartDate < '2019-01-01' THEN DATEDIFF(DAY, '2019-01-01', EndDate)
    ELSE DATEDIFF(DAY, StartDate, EndDate)
  END)+1 AS DaysInRange
FROM Dates


ID
StartDate
EndDate
DaysInRange


1
2016-06-09
2019-12-31
365

2
2016-06-09
2019-01-30
30

3
2019-02-09
2019-07-30
172

ID	StartDate	EndDate	DaysInRange
1	2016-06-09	2019-12-31	365
2	2016-06-09	2019-01-30	30
3	2019-02-09	2019-07-30	172