Custom Events are generally used within our Automation messages to trigger messages when an event occurs or reaches a certain value. It wouldn't necessarily be used as a segmentation option.
If you want to send to a segment of your audience that have performed a particular event within a certain timeframe, the best way to do that would be to use our Insight product.
Within Insight, you can generate a report of users who have or have not performed a particular custom event within a specified timeframe. You can then export that as a list of users that you can then upload and send to within the UI or Urban Airship API.