It is not
It is a bad idea to use the above formula. Why? Because there are cases where it will behave incorrectly and you might snap your keyboard trying to figure out why doesn't your Google Data Studio dashboard show the same data as you can see in your Google Analytics account.
The problem comes from the source dimension used to build your Month of Year field. Since the source dimension is not based on Month but on a single day (Date), the data pulled from your Google Analytics will have different granularity.
This might not be a problem with metrics attached to a single event, however metrics over larger scopes that are less dependent on a single point in time will get distorted results.
The above image demonstrates the difference on the metric Total users. The top chart uses correctly prepared Month of Year dimension and shows visitors corresponding to the numbers in Google Analytics. The lower chart uses incorrect Month of Year dimension that uses Date as source dimension and the number of visitors appears higher.
So what is the reason for this difference? It is simple: The source dimensions you use to build custom fields in Data studio dictate how will the data get collected from Google Analytics database. When you use Date as a base for your Month of Year dimension, the Total users metric will be exported for each day independently and only afterwards merged together to add up to the months. This is obviously a problem - some visitors might have visited your website on multiple days, however their visits are now added up as if they come from different visitors.
So how to build Month of Year correctly? You must stick only to Year and Month dimensions. Merge them into a single field. The following formula is my crude implementation.
PARSE_DATE('%Y-%m', CONCAT(Year, '-', IF(Month < 10, CONCAT('0', Month), CAST(Month AS TEXT ) ) ) )
The Year and Month dimensions are joined as a text and Month is zero padded when necessary (sadly there is no LPAD function in Data studio). Finally, the resulting text is converted back to Date format so Data studio can recognize this is a time-based field.
- Updated bar chart image to corresponding time frames.
- Changed the correct dimension formula from compat. mode TODATE to PARSE_DATE.