SQL Server 2022

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CREATE TABLE Example 
(
    Task	VARCHAR(512),
    UID	VARCHAR(50),
    sessionID	VARCHAR(50),
    logTime	DateTime
);

INSERT INTO Example (Task, UID, sessionID, logTime) VALUES
	('Start',  '170431926600021', '1704319266', '2024-01-03 22:01:06'),
	('Task-1', '170431926600021', '1704319266', '2024-01-03 22:01:07'),
	('Start',  '170433797800098', '1704337978', '2024-01-04 03:12:58'),
	('Task-1', '170433797800098', '1704337978', '2024-01-04  03:12:59'),
	('Start',  '170436928400009', '1704369284', '2024-01-04 11:54:44'),
	('Task-1', '170436928400009', '1704369284', '2024-01-04 11:54:45'),
	('Task-2', '170433797800098', '1704337978', '2024-01-04 16:13:00'),
	('Task-2', '170431926600021', '1704319266', '2024-01-05 19:01:08'),
	('Task-2', '170436928400009', '1704369284', '2024-01-06 00:54:46');

SELECT * FROM Example;

WITH start as
(
SELECT *
FROM Example
WHERE Task='Start'  
),
Calculated as
(  
SELECT e.UID,e.Task, s.logTime as StartTime, e.logTime, isnull(LAG(e.logTime) OVER(PARTITION BY e.UID ORDER BY (e.logTime)),s.logTime) as PreviouslogTime
FROM Example e
INNER JOIN Start s on S.UID=e.uid  
WHERE e.Task!='Start'  
)
SELECT  c.UID,c.Task, c.logTime as StartTime, c.logTime, datediff(second, c.PreviouslogTime, c.logtime) as Elapsed, datediff(second, c.StartTime, c.logtime) as TotalElapsed
FROM  Calculated c

Task	UID	sessionID	logTime
Start	170431926600021	1704319266	2024-01-03 22:01:06.000
Task-1	170431926600021	1704319266	2024-01-03 22:01:07.000
Start	170433797800098	1704337978	2024-01-04 03:12:58.000
Task-1	170433797800098	1704337978	2024-01-04 03:12:59.000
Start	170436928400009	1704369284	2024-01-04 11:54:44.000
Task-1	170436928400009	1704369284	2024-01-04 11:54:45.000
Task-2	170433797800098	1704337978	2024-01-04 16:13:00.000
Task-2	170431926600021	1704319266	2024-01-05 19:01:08.000
Task-2	170436928400009	1704369284	2024-01-06 00:54:46.000

UID	Task	StartTime	logTime	Elapsed	TotalElapsed
170431926600021	Task-1	2024-01-03 22:01:07.000	2024-01-03 22:01:07.000	1	1
170431926600021	Task-2	2024-01-05 19:01:08.000	2024-01-05 19:01:08.000	162001	162002
170433797800098	Task-1	2024-01-04 03:12:59.000	2024-01-04 03:12:59.000	1	1
170433797800098	Task-2	2024-01-04 16:13:00.000	2024-01-04 16:13:00.000	46801	46802
170436928400009	Task-1	2024-01-04 11:54:45.000	2024-01-04 11:54:45.000	1	1
170436928400009	Task-2	2024-01-06 00:54:46.000	2024-01-06 00:54:46.000	133201	133202