Any big brains out there know of a universal way to measure time?

Like let’s say we’ve made contact with an alien race and have figured out communications. We want to tell them a thing will happen at a specific time, but they have a different means of measurement that we can’t understand. Is there a universal constant way to measure time that doesn’t assume a standard planetary rotation, or the reactions of a specific atom to specific conditions? A constant regardless of environmental conditions that with the basic equipment of a reasonably techno-savvy civilization could easily be measured

*edit* Could the hydrogen line frequency be used as constance reference point perhaps?

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Starcrossed · 41-45, F

Measuring the passage of time itself can be done with atomic clock.

Determining what constitutes as a "day" cycle is another matter.

https://www.nasa.gov/missions/tech-demonstration/deep-space-atomic-clock/what-is-an-atomic-clock/

Zaphod42 · 46-50, M

@Starcrossed true, but atomic time is set by the vibrations of a cesium atom irradiated by a very specific microwave frequency. What if the aliens don’t have easy access to cesium, or for whatever reason are unable to use microwaves to make it vibrate at the exact frequency needed?