Find if a date changes

In Power Query, you can add an Index column to refer to the previous row.

Paste the below code into the Advanced Editor to see how it works:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjTQN9Q3MjAyVdJRMje0MDAG0mb6RuYgMXOlWB2wCiOCKowJqjAjqMIcU4WxAUjMAqbCgqAZlgRVGBrgURILAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Run Date" = _t, #"facility code" = _t, #"maturity date" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Run Date", type date}, {"facility code", Int64.Type}, {"maturity date", type date}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(#"Added Index", "Change?", each 
        if [Index] = 0 then "N" 
        else if #"Changed Type"[maturity date]{[Index]-1} = [maturity date] then "N" else "Y", type text)
in
    #"Added Custom"