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 Android Devices:
  1. Open the Google Play Store app Google Play.
  2. At the top right, tap the profile icon.
  3. Tap Manage apps & device and then Manage.
  4. Tap the Lemmings app.
  5. Tap Update.

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

  1. Open the Google Play Store app Google Play.
  2. At the top right, tap the profile icon.
  3. Tap Manage apps & device.
  4. Tap Manage, then find the Lemmings app.
  5. To open the app's "Details" page, tap the app.
  6. On the app's "Details" page, tap More.
  7. Turn on Enable auto update.
Now when updates are available, they will be automatically downloaded.