Maybe what you want is to create a calculation based on the base date, something that looks like this. This will maintain the date format, but it will truncate the detail to the month level.
DATETRUNC(date_part, date, [start_of_week])
Truncates the specified date to the accuracy specified by the date_part. This function returns a new date. For example, when you truncate a date that is in the middle of the month at the month level, this function returns the first day of the month. The
start_of_weekparameter, which you can use to specify which day is to be considered the first day or the week, is optional. Possible values are 'monday', 'tuesday', etc. If
start_of_weekis omitted, the start of week is determined by the data source. See Date Properties for a Data Source.
Supports ISO 8601 dates.
DATETRUNC('quarter', #2004-08-15#) = 2004-07-01 12:00:00 AM
DATETRUNC('month', #2004-04-15#) = 2004-04-01 12:00:00 AM
---->See more ---> Date Functions - Tableau