Yes, in XPath you can use the token [%MonthLength%], see this documentation page. Note that the actual token is not present in the documentation, but the modeller accepts it anyway.
You expression could look something like:
[createdDate >= '[%BeginOfCurrentMonthUTC%] - [%MonthLength%]']
[createdDate <= '[%EndOfCurrentMonthUTC%] - [%MonthLength%]']