Please generate the Calendar for your field, For now - Simplify using this
Date(Date#(FieldName, 'YYYY-MM'))
Or
=MakeDate(Year(Date#(FieldName, 'YYYY-MM')),Month(Date#('2019-09','YYYY-MM')))
Before develop something, think If placed (The Right information | To the right people | At the Right time | In the Right place | With the Right context)