Match date field on day in the month

Hey, I have a date type field that I wont to match by, and get all documents that the date field in them is the first day of the month.
I tried regex but it does not work because it's not a string field.

I will be happy to find an answer (:

You can create a runtime field that returns the day of month. Then use this field for filter for first day of the month. You can also use this field for visualizing data by day month.

This is the script used to create a day of month runtime field from the date field timestamp

emit(doc['timestamp'].value.getDayOfMonth());