How to Log Workouts (iPhone)
You can log a workout from the homescreen by tapping the "START WORKOUT" button.
Alternatively, you can simply select the workout that you want to do from the homescreen. As soon as you enter a set, the workout date will change to today's date.
The date of new workouts is set under homescreen - edit - schedule. You can change the date of new workout inside the workout screen (edit workout - workout date)
If you skip a workout, don't log it. You don't need to log a workout with blank reps or anything similar. Just don't the workout in the first place.
Your calendar will show no workout for that particular day. And the app will properly change the weight if it takes you longer than a week to get back to training.
For more information on what to do if you skip a workout, see this: