Adding Duration to time in previous row within the same currently calculated column

Hello Anonymous 

you can use List.Generate to achive this. Here the example with your data provided

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSAWNLKwMDpVidaCUjIMcIWcAYyDFGFjABckzgArEA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Item No." = _t, Duration = _t, #"Start time" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Item No.", Int64.Type}, {"Duration", Int64.Type}, {"Start time", type time}}),
    #"Sorted Rows" = Table.Sort(#"Changed Type",{{"Item No.", Order.Ascending}}),
    CreateStartEnd = List.Generate
    (
        ()=> [Counter = 0, Start= #"Sorted Rows"[Start time]{0}, End= #"Sorted Rows"[Start time]{0} + #duration(0,#"Sorted Rows"[Duration]{0},0,0)],
        each [Counter]<= Table.RowCount(#"Sorted Rows")-1,
        each 
        [
            Counter = [Counter]+1,
            Start = [End],
            End= [End]+ #duration(0,#"Sorted Rows"[Duration]{[Counter]+1},0,0)
        ],
        each {[Start], [End]}
    ),
    ToRows = Table.FromRows(CreateStartEnd, {"Start", "End"}),
    Combine = Table.FromColumns(Table.ToColumns(#"Sorted Rows")&Table.ToColumns(ToRows),Table.ColumnNames(#"Sorted Rows")&Table.ColumnNames(ToRows))

in
    Combine

Copy paste this code to the advanced editor in a new blank query to see how the solution works.

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Have fun

Jimmy