Pokemon Duel issues

[Pokémon Spectrum]

I just downloaded Pokemon Duel the other day, and it's amazing! However, I've run into an issue; the timer doesn't seem to run while the mobile device is asleep. It's really frustrating, because I had a Timed Booster opening, and last night there were about three hours left. This morning, I turn on my iPod, see the notification that the Booster's ready, open the app... and see that there are still three hours left until I can open it! Even worse, I tried putting it to sleep and turning it back on, and the timer had gone down... and then it jumped back up to how much time was left before I put it to sleep. Does it need a constant Wi-Fi connection to keep running? If so, it's a major oversight on the developer's part...
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Never mind, I restarted my iPod and it fixed the issue, but still, the fact that it happened at all is a rather irritating problem. The only reason I restarted my iPod is because the game itself suggested relaunching the app, but there's no way to actually close the app. They seriously need to fix that, I don't want to have to restart my iPod every time an issue like that comes up. So now my question is, does anybody know how to close the app without turning the whole device off?