One way you can do it is set a Relative Date filter for "last 30 days", and Anchor Relative to: Today. Once you do that you can just do a count-distinct on the user IDs.
That should be all that's needed, though you may need to be mindful of what time your data extract updates if you're using one. I would think closer to midnight would be best. Otherwise you could add few hours uniformly to all your datetimes to change the day-to-day changeover time to be as close to your daily update time as possible... if you really need that level of precision. Otherwise just ignore this second paragraph