Calendar Days Decades

Hello Pillic 

you can add a nestef if-function to achieve this. Here the formula

if [Tag]<11 then 1 else if [Tag]<21 then 2 else 3

If you would not have the day-value in your table, but only a date-value you could substitute the [Tag] with Date.Day([Date]).

here a complete example

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Lc67DcAwDAPRXVSnCKk4n1kM779GAJ6bU8UHzVmqdcxy2umVjvROn/RNv1Qnh7WYi70AhCAIYQhEKEbx/gHFKEYxilGMYhSjNErvX2qtHw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Tag = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Tag", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Decade", each if [Tag]<11 then 1 else if [Tag]<21 then 2 else 3)
in
    #"Added Custom"

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

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Kudoes are nice too

Have fun

Jimmy