This is not possible with XPath, see Fedor's answer. Because it is only possible to use XPath as data source for the datagrid, you have to create a report with an OQL query as data source.
You can use the following OQL query:
SELECT Name AS Name, Birthdate AS Birthdate, DATEPART(DAY, Birthdate) AS DayOfMonth FROM Module.Person WHERE DATEPART(MONTH, Birthdate) = DATEPART(MONTH, '[%CurrentDateTime%]') ORDER BY DayOfMonth
You should define a dataset and model a report pane to show this data.
It is not possible to put the Xpath constraint [Birthdate >= '[%BeginOfCurrentMonth%]'][Birthdate <= '[%EndOfCurrentMonth%]'] on the datagrid. This will only result in a contraint on the months of the current year.
The following microflow expression can also not be used as Xpath constraint on the datagrid: [formatDateTime(BirthDate,'MMM' = formatDateTime('[%CurrentDateTime%]','MMM')]
Probably you should start writing OQL queries to meet your requirements ....
You can set a Xpath constraint on the datagrid as followed:
[birthdate >= '[%BeginOfCurrentMonth%]'][birthdate <= '[%EndOfCurrentMonth%]']