Simple Question - How to get dates to sum into one month?

By the looks of it, you actually didn't change the value of the copied column but changed the format. The underlying value is still the full date (of type, datetype). 

You could create a calculated column with the following dax:

CalcColumnMonthYear = 
VAR month = FORMAT(DATEVALUE("2018-" & Table[originaldatecolumn] & "-1") , "MMM")
VAR year = FORMAT(YEAR(Table[originaldatecolumn]), "####")
RETURN
month&" "*year

This will return a "December 2019" format column as TEXT and then you can indeed expect it to sum the grand total per month. 

To be honest; the best way to do this is to use a datetable, create a relationship from you original date column to that date table and use a date hierarchy in combination with a table visual. I recommend this question and answer: https://community.powerbi.com/t5/Desktop/Date-table-s-why-are-they-needed-and-problems-they-bring-with/td-p/51745 

Alternatively google around on datetables in combination with powerbi and you will find a ton of blogs on how to use a datetable to get your desired result! 🙂

Kind regards

Djerro123

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