Tbf, seconds are defined by a fundamental physical property already, the properties of a caesium atom because that was the most accurate way of keeping time known. Seconds defined this way are part of the SI metric standard. If we were to meet ET comrades, so long as we could communicate what hydrogen is, which should be pretty easy, we could extrapolate what our time measurements are based on from there... probably more easily than using planck multiples, which to my knowledge has nothing in the physical world that oscillates by it. (Also, it's no better or worse than using any other multiple, but using multiples of 10 is arbitrary because of our base-10 system that we use as a result of having 10 fingers. Who knows how many fingers ET has!) 👽

If we're basing our units on something to appease potential aliens, I personally like the idea of using pulsars, which when discovered were thought at first to be aliens because they pulsed with frequencies more accurate than our atomic clocks.

Honestly, the dolphins should be making these decisions regardless.