difference between sql datediff code example

Example: sql server query for datediff

-- DOES NOT ACCOUNT FOR LEAP YEARS
DECLARE @date1 DATETIME, @date2 DATETIME, @result VARCHAR(100);
DECLARE @years INT, @months INT, @days INT,
    @hours INT, @minutes INT, @seconds INT, @milliseconds INT;

SET @date1 = '1900-01-01 00:00:00.000'
SET @date2 = '2018-12-12 07:08:01.123'

SELECT @years = DATEDIFF(yy, @date1, @date2)
IF DATEADD(yy, -@years, @date2) < @date1 
SELECT @years = @years-1
SET @date2 = DATEADD(yy, -@years, @date2)

SELECT @months = DATEDIFF(mm, @date1, @date2)
IF DATEADD(mm, -@months, @date2) < @date1 
SELECT @months=@months-1
SET @date2= DATEADD(mm, -@months, @date2)

SELECT @days=DATEDIFF(dd, @date1, @date2)
IF DATEADD(dd, -@days, @date2) < @date1 
SELECT @days=@days-1
SET @date2= DATEADD(dd, -@days, @date2)

SELECT @hours=DATEDIFF(hh, @date1, @date2)
IF DATEADD(hh, -@hours, @date2) < @date1 
SELECT @hours=@hours-1
SET @date2= DATEADD(hh, -@hours, @date2)

SELECT @minutes=DATEDIFF(mi, @date1, @date2)
IF DATEADD(mi, -@minutes, @date2) < @date1 
SELECT @minutes=@minutes-1
SET @date2= DATEADD(mi, -@minutes, @date2)

SELECT @seconds=DATEDIFF(s, @date1, @date2)
IF DATEADD(s, -@seconds, @date2) < @date1 
SELECT @seconds=@seconds-1
SET @date2= DATEADD(s, -@seconds, @date2)

SELECT @milliseconds=DATEDIFF(ms, @date1, @date2)

SELECT @result= ISNULL(CAST(NULLIF(@years,0) AS VARCHAR(10)) + ' years,','')
     + ISNULL(' ' + CAST(NULLIF(@months,0) AS VARCHAR(10)) + ' months,','')    
     + ISNULL(' ' + CAST(NULLIF(@days,0) AS VARCHAR(10)) + ' days,','')
     + ISNULL(' ' + CAST(NULLIF(@hours,0) AS VARCHAR(10)) + ' hours,','')
     + ISNULL(' ' + CAST(@minutes AS VARCHAR(10)) + ' minutes and','')
     + ISNULL(' ' + CAST(@seconds AS VARCHAR(10)) 
     + CASE
            WHEN @milliseconds > 0
                THEN '.' + CAST(@milliseconds AS VARCHAR(10)) 
            ELSE ''
       END 
     + ' seconds','')

SELECT @result