Calculate Datediff between rows in one column (with more than 2 records)

Question

I have a table which has a column of dates which I want to find the difference between. I have been able to find off your site an answer that gets me the difference between

Answer 1

when i treid this i am getting error like: Cannot find either column "Timelapse" or the user-defined function or aggregate "Timelapse.start_date_time", or the name is ambiguous.

i replcaed 'AllDays' with 'Timelapse'

SELECT
Timelapse.[request_id],
--Timelapse.[task_code],
Timelapse.[start_date_time]-->getting error at this column
(DateDiff(day,[PreviousDate],[start_date_time]), 0) AS Days_Between
FROM
    (
        SELECT
            [request_id],[task_code],
            [start_date_time],
            (
                SELECT Max([start_date_time])
                FROM [Monitor247].[dbo].[request_task] AS T2
                WHERE
                        T2.[request_id] = T1.[request_id]
                    AND T2.[start_date_time] < T1.[start_date_time]
            ) AS PreviousDate
        FROM [Monitor247].[dbo].[request_task] AS T1
    ) AS Timelapse;