Date Format in M-Query

Hi community,

I've a doubt regarding Date Formatting in M-Query.

I'm trying to learn and execute Date formatting in M-Query using below M-Query code:

Table.TransformColumns(Source, {{"DateColumn", each DateTime.ToText(_, "yyyy-MM-dd"), type text}})

I'm doing the transforming in Calendar Date Master Table.

I've Date and Year column and based on the Date column I'm trying to Create a Custom column  named (Custom Date Format) using M-Query.

Below is the error snip:

Please help!