datediff

https://docs.microsoft.com/en-us/sql/t-sql/functions/datediff-transact-sql

DATEDIFF ( datepart , startdate , enddate )  

返回值是，用结束时间，减去开始时间

CREATE TABLE dbo.Duration  
    (  
    startDate datetime2  
    ,endDate datetime2  
    );  
INSERT INTO dbo.Duration(startDate,endDate)  
    VALUES('2007-05-06 12:10:09','2007-05-07 12:10:09');  
SELECT DATEDIFF(day,startDate,endDate) AS 'Duration'  
FROM dbo.Duration;  
-- Returns: 1