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?