Create start and end date columns from rows

One approach would be as follows.

1. Add a column that returns the 'Start Date' for all rows that have a Milestone that does not end with 'End'
2. then add a column that returns the 'Start Date' for rows that do not end in 'Start'
3. on the second created column 'Fill Up' to fill in the null values
4. remove the 'Start' or 'End' from any rows in the 'MIlestone' column that have those as ending words
5. filter out null values from the first created column
6. remove the original 'Start Date' column
7. rename created columns as needed

As an example, starting with...

and ending up with...

with the code...

let
    Source = 
    Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("lZKxCoMwFEV/RTILaV6M2LFIC91ER3EIKjQgEUKkv19FWiE28blkeffm5YRT10SkQGKSl8V8cspSChcA0sTfSfkoonwyptc2qtpX301DH1VWGjsPGaOwFPhR4a47T7waB9UqK60adWARFeeK60KgiVu7vaXpdvl5ICjjSzhxWJ5aWSUHPPyugIT3LzqA9yz0wrv5HTzLxM+JjMJ1u2adnHAiUPj3LWsc6wS4T8Y5wdwa0omNBelEoICEDzgRhvc64YEPO9F8AA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ProjectID = _t, Milestone = _t, #"Start Date" = _t]),
    changeDataTypes = 
    Table.TransformColumnTypes(Source,{{"ProjectID", Int64.Type}, {"Milestone", type text}, {"Start Date", type date}}),
    addStartDate = 
    Table.AddColumn(changeDataTypes, "sDate", each if not Text.EndsWith([Milestone], "End") then [Start Date] else null, type date),
    addEndDate = 
    Table.AddColumn(addStartDate, "eDate", each if Text.EndsWith([Milestone], "Start") then null else [Start Date], type date),
    fillUpEndDate = 
    Table.FillUp(addEndDate,{"eDate"}),
    cleanMilestones = 
    Table.TransformColumns(fillUpEndDate, {{"Milestone", each if Text.EndsWith(_, "Start") or Text.EndsWith(_, "End") then Text.BeforeDelimiter(_, " ", RelativePosition.FromEnd) else _, type text}}),
    filterRows = 
    Table.SelectRows(cleanMilestones, each ([sDate] <> null)),
    removeOriginalDate = 
    Table.RemoveColumns(filterRows,{"Start Date"}),
    renameColumns = 
    Table.RenameColumns(removeOriginalDate,{{"sDate", "Start Date"}, {"eDate", "End Date"}})
in
    renameColumns