Need help with my first custom function

Hi klehar 

This step is to convert the original date value into a table. For table functions (like Table.RenameColumns, Table.TransformColumnTypes, Table.AddColumn...), the first argument should always be a table rather than other types. 

You could modify Pat's code into below and provide a specific date for Source step. In this way, it returns a table. You can add columns to it. After you have added all columns, remove "//" in the code and remove the Source=#date... step. Then it will be converted to a custom function again. 

// (DateInput as date) =>
let
// Source = DateInput,
Source = #date(2021,1,1),
#"Converted to Table" = #table(1, {{Source}}),
#"Renamed Columns" = Table.RenameColumns(#"Converted to Table",{{"Column1", "Date"}}),
#"Changed Type" = Table.TransformColumnTypes(#"Renamed Columns",{{"Date", type date}}),
#"Inserted Month Name" = Table.AddColumn(#"Changed Type", "Month Name", each Date.MonthName([Date]), type text),
#"Inserted Year" = Table.AddColumn(#"Inserted Month Name", "Year", each Date.Year([Date]), Int64.Type)
in
#"Inserted Year"

You may find below articles helpful. 

Create Calendar Table Using Power Query M Language (mssqltips.com)

Build a Reusable Calendar Table with Power Query - Excelerator BI

Best Regards,
Community Support Team _ Jing
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