Please remember that the 20 days is to serve the response, meaning mailing it, not to file it. So, the earliest you can file the Motion would be the 21st day, but as a practical matter I file on the 22nd day. The Clerk isn't going to enter a default on the 21st day anyway; at least not in my area.
Don't forget to prepare the Affidavit of Non-Military Status--that is something you can do now, you do not have to wait to the 20th day to do.
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The other lawyers are correct, and by the way, if the default is entered, it will be very easy to vacate. Especially on a time sharing issue, the court may want to make sure he is heard and would likely grant a motion to vacate, if he is just late. If he ignores it, that is something else. But give it a few days so you're not wasting your time.
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