I do not think we have a way currently in the API to look up a set of users who have logged in over the last X number of days (nor by any other time-unit). The closest thing I think we have is a V2 call that tells you the last visit time for each user:
https://mycommunity.mydomain.com/api/2.0/search?q=SELECT+ last_visit_time+FROM+users
You can add any of these fields to the WHERE clause of your LiQL users query.
That list currently does not contain the last_visit_time field, but I could see us wanting to add something like that in the future. I would recommend posting an idea in our product ideas board requesting that the last_visit_time be added as a LiQL constraint so you can search for users that have signed in over the last 30 days.
-Doug