I have built a Watchkit application in Swift that includes a timer (countdown from 25 minutes by seconds) and I want to be able to display that countdown in the iOS app.
The problem is, the method that updates the timer is in the Apple Watch app, and I have no clue how to access this from the parent iOS app.
You are going to need to have your watch app alert the iOS app and inform it of how much time there is remaining in the timer (best done by sending a timestamp [since epoch]).
The simplest way to do this would be to call the openParentApplication:reply method of the WKInterfaceController as outlined in this answer.
Another way would be to store that timestamp info so that the next time the iOS app is opened, it can fetch the info. This can be done via making a custom NSUserDefaults to store data to as outlined in this answer.
If you want the information to be updated realtime (make a change on the watch and have it update on the iPhone and vice versa), use MMWormhole, a really nice open-source library that deals will encapsulating the headaches of working with Darwin notifications.