Unless I were able to know your exact reference frame so that I could calculate the time dilation, scheduling doesn’t do you or I any good.

Over everyday distances and speeds, this effect is tiny and entirely negligible.

Any difference between two living people on Earth (disregarding time zones for the moment) will be far less than 1 second, and I think we can deal with it if one of us shows up 1 second early or late to lunch.

No, we do not need to do any relativistic calculations in order to set a time to meet for lunch.

So really what we need is a way to measure energy states. An energy clock. The universe is relaxing, perpetually over time. That total energy, therefore, is observable in all reference frames. So if we want to schedule something effectively, we set an energy state to match.

Great … so we’ll just both need an array of high-powered telescopes and a supercomputer in order to figure out when to meet each other for lunch.