In English, the phrase “on time” can refer to something happening at the expected or scheduled time, without delay. In French, there are two main ways to translate “on time”: à temps and à l’heure. They are not interchangeable. The correct choice depends on the context.

À l’heure

The phrase à l’heure is used when talking about being punctual, especially with reference to a clock or a schedule. It applies when someone arrives, departs, or does something at the correct or expected time.

Examples:

• Il est toujours à l’heure pour ses rendez-vous.
  He is always on time for his appointments.
• Le train est arrivé à l’heure.
  The train arrived on time.
• Je veux que tu sois à l’heure demain matin.
  I want you to be on time tomorrow morning.
• Elle n’est pas souvent à l’heure.
  She is not often on time.
• Merci d’être venu à l’heure.
  Thank you for coming on time.

À l’heure refers to clock-based timing. It is often used when someone arrives or something happens at the time fixed by a timetable or plan.

À temps

The phrase à temps means “in time” or “before it is too late.” It is used when referring to actions completed early enough to avoid a problem, meet a deadline, or prevent something negative.

Examples:

• J’ai terminé mon devoir à temps.
  I finished my assignment on time.
• Est-ce qu’on arrivera à temps pour le spectacle ?
  Will we arrive on time for the show?
• Heureusement, ils sont arrivés à temps pour sauver le chien.
  Fortunately, they arrived in time to save the dog.
• Tu dois payer tes impôts à temps.
  You must pay your taxes on time.
• Si tu veux réussir, tu dois rendre ton travail à temps.
  If you want to succeed, you must hand in your work on time.

À temps focuses more on a deadline or a time limit, not a scheduled clock time. It implies that the action was completed before it became too late.

Choosing between à l’heure and à temps

Use à l’heure when referring to being punctual according to a schedule.
Use à temps when referring to completing something early enough to avoid negative consequences.

Compare:

• Il est arrivé à l’heure à la réunion.
  He arrived at the meeting on time (punctually).
• Il est arrivé à temps pour éviter une dispute.
  He arrived in time to avoid an argument.

Although both may be translated as “on time,” the nuance is different. À l’heure is about the expected moment; à temps is about being early enough to achieve or prevent something.