I moved the thread from Dynamics AX forum to Dynamics 365 Finance forum and set the category to Development / Customization / SDK.
Don't create the enum - I'm pretty sure that it already exists.
What code you should write in the DP class depends on what you want it to do. You need to deside it first. For example, let's say that you have a single field called TransDate and you want to filter by this field. If so, a possible approach is using TransDate >= [first day of the month && TransDate <= [last day of the month.
You can calculate the dates by something like this:
date fromDate = mkDate(1, month, year(systemDateGet());
date toDate = endMth(fromDate);
How you should apply the range depends on the way how you select the data - either by a while select statement or Query* classes.
It'll be much easier to give you an answer if you tell us more about what you want.
