Date / Time difference excluding weekends and factoring working hours

Check this. Ignore first 4 rows (I've added them just for test purpose).

You can edit shiftStart and shiftEnd in Helper step.

EDIT: I've added option where you can decide whether you want to include or exclude weekends:

Output

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("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", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [début = _t, dateEC = _t, DiffEC = _t, dateT = _t, dateF = _t]),
    ChangedType = Table.TransformColumnTypes(Source,{{"dateF", type datetime}, {"dateT", type datetime}, {"DiffEC", type duration}, {"dateEC", type datetime}, {"début", type datetime}}),
    Helper = [ shiftStart = #time(8,0,0),
    shiftEnd = #time(17,0,0),
    includeWeekends = "no", //type "yes" if you want to include weekends. Type "no" if you want to exclude weekends
    workingHours = shiftEnd - shiftStart ],
    fn_WorkingDuration = 
        (previousStep as table, start as text, end as text, optional colName as text)=>
        let
            // Ad_StartColumn = Table.DuplicateColumn(ChangedType, "début", "S"),
            // Ad_EndColumn = Table.DuplicateColumn(Ad_StartColumn, "dateEC", "E"),
            Ad_StartColumn = Table.DuplicateColumn(previousStep, start, "S"),
            Ad_EndColumn = Table.DuplicateColumn(Ad_StartColumn, end, "E"),

            Ad_SDate = Table.AddColumn(Ad_EndColumn, "S Date", each DateTime.Date([S]), type date),
            Ad_STime = Table.AddColumn(Ad_SDate, "S Time", each DateTime.Time([S]), type time),
            Ad_EDate = Table.AddColumn(Ad_STime, "E Date", each DateTime.Date([E]), type date),
            Ad_ETime = Table.AddColumn(Ad_EDate, "E Time", each DateTime.Time([E]), type time),
            // Weekends excluded
             Ad_WorkingDays = Table.AddColumn(Ad_ETime, "Working Days", each try 
                [ a = List.Dates([S Date], Duration.TotalDays([E Date] - [S Date]) +1, #duration(1,0,0,0)), //weekends included
                  b = List.Select(a, (x)=> Date.DayOfWeek(x, Day.Monday) < 5), //weekends excluded
                  c = if Text.Trim(Text.Lower(Helper[includeWeekends])) = "yes" then a else b
                ][c] otherwise null, type list),
            // [S Date] and [E Date] excluded
            Ad_FullDaysHours = Table.AddColumn(Ad_WorkingDays, "Full Days Hours", each try
                [ a = List.RemoveItems([Working Days], { [S Date], [E Date]}),
                b = Helper[workingHours] * List.Count(a)
                ][b] otherwise null, type duration),
            Ad_FirstDayHours = Table.AddColumn(Ad_FullDaysHours, "Fist Day Hours", each try if [S Date] = List.First([Working Days])
                then (if [S Date] = [E Date] then List.Min({ [E Time], Helper[shiftEnd] }) else Helper[shiftEnd]) - List.Max({ [S Time], Helper[shiftStart] })        
                else #duration(0,0,0,0) otherwise null, type duration),
            Ad_LastDayHours = Table.AddColumn(Ad_FirstDayHours, "Last Day Hours", each try if [S Date] = [E Date] then #duration(0,0,0,0) else if [E Date] = List.Last([Working Days])
                then Helper[workingHours]        
                else #duration(0,0,0,0) otherwise null, type duration),

            // Ad_TotalWorkingDuration = Table.AddColumn(Ad_LastDayHours, "Total Working Duration", each [Full Days Hours] + [Fist Day Hours] + [Last Day Hours], type duration),
            Ad_TotalWorkingDuration = Table.AddColumn(Ad_LastDayHours, if colName <> null then colName else "Total Working Duration" , each [Full Days Hours] + [Fist Day Hours] + [Last Day Hours], type duration),

            _RemovedColumns = Table.RemoveColumns(Ad_TotalWorkingDuration,{"S", "E", "S Date", "S Time", "E Date", "E Time", "Working Days", "Full Days Hours", "Fist Day Hours", "Last Day Hours"})
        in
            _RemovedColumns,
    StepBack = ChangedType,
    Ad_dateECDuration = fn_WorkingDuration(StepBack, "début", "dateEC", "dateEC duration"),
    Ad_dateTDuration = fn_WorkingDuration(Ad_dateECDuration, "début", "dateT", "dateT duration"),
    Ad_dateFDuration = fn_WorkingDuration(Ad_dateTDuration, "début", "dateF", "dateF duration")
in
    Ad_dateFDuration