[LASR]

> "sunrise in Singapore" or "astronomical noon".

Yes exactly. But how would you measure astronomical noon? You might build some instruments that look at the sky and produce some readings.

Now you might build multiple copies of your instrument and compare their readings. But they disagree by some amount.

So you build a better instrument. Or choose some other thing to measure instead. Rinse repeat until you have a more precise instrument.

Atomic clocks are instruments that are measuring something in the universe. They are not generating some time stamp out of nothing. The thing that you say as an objective reference still very much applies.