How does this Time Difference Calculation work? - tsql

Question

I wanted to display the difference in HH:MM:SS between two datetime fields in SQL Server 2014.
I found a solution in this Stack Overflow post. And it works perfectly. But I want to understand the "why" of how this arrives at the correct answer.
T-SQL:
SELECT y.CustomerID ,
y.createDate ,
y.HarvestDate ,
y.DateDif ,
DATEDIFF ( DAY, 0, y.DateDif ) AS [Days] ,
DATEPART ( HOUR, y.DateDif ) AS [Hours] ,
DATEPART ( MINUTE, y.DateDif ) AS [Minutes]
FROM (
SELECT x.createDate - x.HarvestDate AS [DateDif] ,
x.createDate ,
x.HarvestDate ,
x.CustomerID
FROM (
SELECT CustomerID ,
HarvestDate ,
createDate
FROM dbo.CustomerHarvestReports
WHERE HarvestDate >= DATEADD ( MONTH, -6, GETDATE ())
) AS [x]
) AS [y]
ORDER BY DATEDIFF ( DAY, 0, y.DateDif ) DESC;
Results:
1239090 2017-11-07 08:51:03.870 2017-10-14 11:39:49.540 1900-01-24 21:11:14.330 23 21 11
1239090 2017-11-07 08:51:04.823 2017-10-19 11:17:48.320 1900-01-19 21:33:16.503 18 21 33
1843212 2017-10-27 19:14:02.070 2017-10-21 10:49:57.733 1900-01-07 08:24:04.337 6 8 24
1843212 2017-10-27 19:14:03.057 2017-10-21 10:49:57.733 1900-01-07 08:24:05.323 6 8 24
The first column in Customer ID - the second and third columns are the columns I wanted to calculate the time difference between. The third column is the difference between the two columns - and one of the points in the code in which I do not understand.
If you subtract two datetime fields like this create date - harvestdate, why does it default to the year 1900?
And regarding DATEDIFF ( DAY, 0 , y.DateDiff) - what does the 0 mean? Does the 0 set the date as '01-01-1900'?
It works - for that I am grateful. I was hoping I could get an explanation as to why this behavior works?

Answer 1

I've added some comments that should explain it:
SELECT y.CustomerID ,
y.createDate ,
y.HarvestDate ,
y.DateDif ,
DATEDIFF ( DAY, 0, y.DateDif ) AS [Days] , -- calculates the number of whole days between 0 and the difference
DATEPART ( HOUR, y.DateDif ) AS [Hours] , -- the number of hours between the two dates has already been cleverly
-- calculated in [DateDif], therefore, all that is required is to extract
-- that figure using DATEPART
DATEPART ( MINUTE, y.DateDif ) AS [Minutes] -- same explanation as [Hours]
FROM (
SELECT x.createDate - x.HarvestDate AS [DateDif] , -- calculates the difference expressed as a datetime;
-- 0 is '1900-01-01 00:00:00.000' as a datetime, so the
-- resulting datetime will be that plus the difference
x.createDate ,
x.HarvestDate ,
x.CustomerID
FROM (
SELECT CustomerID ,
HarvestDate ,
createDate
FROM dbo.CustomerHarvestReports
WHERE HarvestDate >= DATEADD ( MONTH, -6, GETDATE ())
) AS [x]
) AS [y]
ORDER BY DATEDIFF ( DAY, 0, y.DateDif ) DESC;