How to reproduce the problem:
1) Create or edit a Gift Exchange or Prompt Meme
2) In Challenge Settings, tick "Sign-up open?
3) Leave "Sign-up opens:" blank
4) Choose a date for "Sign-up closes:"
5) Choose "Submit" (or "Update", if you're editing an existing challenge)
What you expect to have happen:
The challenge profile should now show "Sign-up: Open" and "Sign-up Closes:" with the chosen closing date (open challenges do not show the "Sign-up Opens:" date).
What does happen:
What should happen instead:
The challenge should be successfully created.