michikah Both the Plank and Horizontal plank have seconds enabled by default. Are you on the latest version of the app? Horizontal plank used to be referenced as repetitions by mistake in a previous version. I am not aware of that issue with any other Plank exercise so please let me know if there is another one.

Note that you can always change the measurement unit yourself on-the-fly by editing the exercise parameters and selecting Time instead of Repetitions.