Short answer: a phone chess clock can be practical for a casual game or club practice when the players and organiser agree to use it. It should not be presented as a certified replacement for equipment required by a tournament. The repository and product plan document 1Chess Clock as an Android chess clock for games at the board; they do not document tournament certification, arbiter approval, or compliance with a particular event's equipment policy.
The short answer
Use the app when the goal is a shared timer for two players and the setting is flexible. It can be useful for home games, coaching, a club training session, or a practice event where the person in charge has explicitly accepted a phone. Start with the getting-started guide, then check the display and turn hand-over before the first serious game.
Where a phone clock can fit
A phone is convenient when you already have one device at the board and want to set a common time, Fischer increment, or Bronstein delay. It also gives the players a simple way to pause and resume during a practice session. That convenience is a use case, not a promise that every organiser will accept a phone. Keep the device stable, charged, and visible to both players. If the event has a supplied clock, use that clock unless the organiser says otherwise.
Why it is not a tournament certification
1Chess Clock has no claim in this guide to be tournament-certified. Rules, equipment policies, noise requirements, backup procedures, and the arbiter's decisions belong to the organiser of the event. A phone may be disallowed, distracting, difficult to secure, or unsuitable for the event's workflow even when the app works correctly. Do not infer approval from the fact that the app can count down at a chessboard.
A practical pre-game checklist
Ask the organiser before the round, not after the clock has started. Confirm that a phone is allowed, whether sound or haptic alerts are acceptable, where the device should sit, and what happens if it loses power or is tapped accidentally. Charge the phone, enable the agreed mode, and run a short test with both players. The long-game setup checklist covers placement and interruption planning; the troubleshooting guide covers sound, pause, and reset checks.
If the organiser says yes
Treat the approval as specific to that event or practice session. Keep the phone on the clock screen, do not open unrelated apps during play, and follow the organiser's procedure for pauses or disputes. If anything about the setup is unclear, stop and ask the organiser. For an official event that requires an approved physical clock, bring and use the required equipment instead of relying on a phone.