如何整合iOS中的拍卖应用倒计时

I am building an app that lets users buy and sell items through the bidding/ auction process. When a user taps on a specific item like a shirt for instance, they will see a countdown of the time remaining to bid on that item. How do i persist the countdown on backend? It seems expensive to persist/update the time remaining for each item, each second to backend (Firebase), and then observe the changes in countdown value to update view. How do apps with auctions like Artsy accomplish this? Thank you!I am building an app that lets users buy and se