If the newest season has not started on your device, and the "ENDS IN:" timer is stuck on 0 days remaining, you are likely missing the latest update to the game. You will need to check for any available updates by following the steps below.
 
For Apple Devices:
  1. Open the App Store.
  2. Tap your profile icon at the top of the screen.
  3. Tap Update next to the Lemmings app.

You can also set the game to update automatically to avoid this issue in the future:

  1. Go to Settings.
  2. Tap App Store.
  3. Turn on App Updates.