USE tempdb;

CREATE TABLE dbo.EngineerTime(
EngineerTimeID int NOT NULL IDENTITY PRIMARY KEY,
FirstName varchar(10) NOT NULL,
StartTime datetime NOT NULL,
EndTime datetime NOT NULL
);

INSERT dbo.EngineerTime VALUES('Lorenzo', CURRENT_TIMESTAMP, DATEADD(minute, 5, CURRENT_TIMESTAMP));
WAITFOR DELAY '00:00:12';
INSERT dbo.EngineerTime VALUES('Lorenzo', CURRENT_TIMESTAMP, DATEADD(minute, 2, CURRENT_TIMESTAMP));
WAITFOR DELAY '00:01:00';
INSERT dbo.EngineerTime VALUES('Lorenzo', CURRENT_TIMESTAMP, DATEADD(minute, 8, CURRENT_TIMESTAMP));
WAITFOR DELAY '00:00:28';
INSERT dbo.EngineerTime VALUES('Andrea', CURRENT_TIMESTAMP, DATEADD(minute, 12, CURRENT_TIMESTAMP));
WAITFOR DELAY '00:00:45';
INSERT dbo.EngineerTime VALUES('Andrea', CURRENT_TIMESTAMP, DATEADD(minute, 1, CURRENT_TIMESTAMP));
WAITFOR DELAY '00:02:16';
INSERT dbo.EngineerTime VALUES('Gianluca', CURRENT_TIMESTAMP, DATEADD(minute, 23, CURRENT_TIMESTAMP));

SELECT *
FROM dbo.EngineerTime;

/* Output:

EngineerTimeID FirstName  StartTime               EndTime
-------------- ---------- ----------------------- -----------------------
1              Lorenzo    2007-10-22 10:48:49.263 2007-10-22 10:53:49.263
2              Lorenzo    2007-10-22 10:49:01.263 2007-10-22 10:51:01.263
3              Lorenzo    2007-10-22 10:50:01.263 2007-10-22 10:58:01.263
4              Andrea     2007-10-22 10:50:29.263 2007-10-22 11:02:29.263
5              Andrea     2007-10-22 10:51:14.263 2007-10-22 10:52:14.263
6              Gianluca   2007-10-22 10:53:30.263 2007-10-22 11:16:30.263

(6 row(s) affected)

*/

WITH CTE_SecondsPerUser([User], Seconds) AS
(
    SELECT FirstName, SUM(DATEDIFF(SECOND, StartTime, EndTime))
    FROM dbo.EngineerTime
    GROUP BY FirstName
)
SELECT
      [User]
    , (Seconds / 86400) AS Days
    , (Seconds / 3600) - (Seconds / 86400 * 24) AS Hours
    , (Seconds / 60) - (Seconds / 86400 * 1440) - ((Seconds / 3600) - (Seconds / 86400 * 24) * 60) AS Minutes
    , (Seconds % 60) AS Seconds

FROM CTE_SecondsPerUser;

/* Output:

User       Days        Hours       Minutes     Seconds
---------- ----------- ----------- ----------- -----------
Andrea     0           0           13          0
Gianluca   0           0           23          0
Lorenzo    0           0           15          0

(3 row(s) affected)

*/

DROP TABLE dbo.EngineerTime;