SELECT DATEDIFF(MI,'2012-07-21 20:32:50','2012-07-22 14:13:48')

ALTER FUNCTION [dbo].[f_time_to_string] (@minutes int)
RETURNS varchar(255)
AS
BEGIN
    DECLARE 
        @days   int,
        @hours  int,
        @result varchar(255)
    
    SET @result = '.'
    IF @minutes < 0 SET @result = '-'
    SET @minutes = ABS(@minutes)
    
    SET @days = @minutes / (24 * 60)
    SET @minutes = @minutes - @days * 24 * 60
    SET @hours = @minutes / 60
    SET @minutes = @minutes - @hours * 60
    
    SET @result = @result + CAST(@days AS varchar(255)) + ' дн.'
    SET @result = @result + RIGHT('00' + CAST(@hours AS varchar(2)), 2) + ':' 
    SET @result = @result + RIGHT('0' + CAST(@minutes AS varchar(2)), 2)
    RETURN @result
END

GO
SELECT dbo.f_time_to_string(75)

SELECT cast(0 as datetime) + cast('2012-07-21 03:00:00' as datetime) - cast('2012-07-22 01:05:00' as datetime)

SELECT cast(cast(cast('2012-07-21 03:00:00' as datetime) as float) - cast(cast('2012-07-22 01:05:00' as datetime) as float) as datetime)

DECLARE @date_begin smalldatetime
DECLARE @date_end smalldatetime

SET @date_begin='20120719 12:33.20'
SET @date_end=  '20120721 15:55.20'

DECLARE @date_b1 int
DECLARE @date_e1 int
DECLARE @date_df int

SELECT @date_b1 = datediff(MI,(0),@date_begin )
SELECT @date_e1 = datediff(MI,(0),@date_end )
SELECT @date_df = datediff(MI,(0),@date_end ) -datediff(MI,(0),@date_begin )

--SELECT @date_e1 - @date_b1 as Itog          -- всего минут, разница между датами
--,(@date_e1 - @date_b1)/1440 as d            -- 60*24 (дней)
--,((@date_e1 - @date_b1)% 1440)/60 as h      -- часов
--,(@date_e1 - @date_b1) - 1440*((@date_e1 - @date_b1)/1440)  
--			- 60*(((@date_e1 - @date_b1)% 1440)/60) as m      -- мин


SELECT @date_df as Itog        -- всего минут, разница между датами
,@date_df/1440 as d            -- 60*24 (дней)
,(@date_df % 1440)/60 as h      -- часов
,@date_df - 1440*(@date_df/1440)  
			- 60*((@date_df  % 1440)/60) as m       -- мин

DECLARE @D1 DATETIME;
DECLARE @D2 DATETIME;
SET @D1='2012-07-21 20:32:50';
SET @D2='2012-07-22 14:13:48';
SELECT STR(s/3600/24)+' дней '+REPLACE(STR(s/3600%24,2)+':'+STR(s/60%60,2)+':'+STR(s%60,2),' ','0')
FROM(SELECT DATEDIFF(SECOND,@D1,@D2))T(s);

declare @firstDate datetime = '2012-07-21 02:00:00',
        @lastDate datetime = '2012-07-22 01:01:07';


select --compute cout of days
       datediff(day, @firstDate, @lastDate) 
       +
       case
        when cast(@lastDate as time) < cast(@firstDate as time)
        then -1
        else 0
       end as cntDays,
       
       --Compute time inteval
       cast(@lastDate - @firstDate as time) as timeInterval

declare @firstDate datetime = '2012-07-21 02:00:00';

select dateadd(day, -cast(@firstDate as int), @firstDate) m1,
       @firstDate - cast(@firstDate as int) as m2,
       dateadd(day, -datediff(day, 0, @firstDate), @firstDate) m3

declare @firstDate datetime 
set @firstDate ='2012-07-21 02:50:00';
select dateadd(day, -cast(@firstDate as int), '20120722 03:45:00') m1

declare @firstDate datetime 
set @firstDate ='2012-07-21 02:50:00';
select dateadd(day, -cast(@firstDate as int), @firstDate) m1