So, you turn the dial on your little radiation machine until you hit the sweet spot that excites the cesium atoms, then you count 9,192,631,770 wobbles of that radiation.
We apologize for the delay, but as some of you are aware, we have had to conduct a cesium test to verify the authenticity of the two bottles we have been presented with.
And so you tell the asteroidaliens that a day is - more or less - an invented time period that is 24 hours long, where each hour is 33 trillion oscillations of a special kind photon emitted by a cesium atom.