How to add 2 date columns into a 3rd column.

Here is one way to do it.

The key was to convert dates to numbers.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjTQN9Q3MlTSAbGMwcxYHaCwIVzYUN/YAC5sBBc2gqmOBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date1 = _t, Date2 = _t]),
    ToDate = Table.TransformColumnTypes(Source,{{"Date1", type date}, {"Date2", type date}}),
    #"Changed Type" = Table.TransformColumnTypes(ToDate,{{"Date2", type number}, {"Date1", type number}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Date3", each ([Date1]+[Date2])/2, type number),
    #"Changed Type1" = Table.TransformColumnTypes(#"Added Custom",{{"Date1", type date}, {"Date2", type date}, {"Date3", type date}})
in
    #"Changed Type1"