Open Durance and tap the plus icon. Give the counter a date, an optional label, and choose whether it counts up (days since that date) or down (days until it). Save, and it appears on your home screen inside the app straight away.

You do not need an account to do this in the phone app — a new counter is saved to your device the moment you add it. (The web app at use.durance.app does need an account.)