Need Help in Formula IF(A1=A2,AC1,AB2)

That's perfect, thanks.

You need to sort the data in the order that you want it to be evaluated, add an index column starting from zero, then add a new custom column with the following calculation:

try
if [Document Id] = previousStepName[Document Id]{[Index] + 1} then [#"Actual End Date & Time"] else previousStepName[#"Actual Start Date & Time"]{[Index] + 1}
otherwise null

Example working query:

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 [#"Document Id" = _t, #"Actual Start Date & Time" = _t, #"Actual End Date & Time" = _t, #"Entity Start Date" = _t]),
    chgTypes = Table.TransformColumnTypes(Source,{{"Document Id", Int64.Type}, {"Actual Start Date & Time", type datetime}, {"Actual End Date & Time", type datetime}, {"Entity Start Date", type datetime}}),

    sortRows = Table.Sort(chgTypes,{{"Document Id", Order.Ascending}, {"Actual Start Date & Time", Order.Ascending}}),
    addIndex0 = Table.AddIndexColumn(sortRows, "Index", 0, 1, Int64.Type),
    addCalcStartDate =
    Table.AddColumn(addIndex0, "calcStartDate", each
        try
            if [Document Id] = addIndex0[Document Id]{[Index] + 1}
            then [#"Actual End Date & Time"]
            else addIndex0[#"Actual Start Date & Time"]{[Index] + 1}
        otherwise null
    )

in
    addCalcStartDate

Example output:

Pete