[remram]

You would want some global reference time, and that could be a number of seconds passing on Earth. However locally you can't work with a clock that doesn't tick at a constant rate (if seconds on your spaceship go twice as fast, the crew can work with that; if the rate changes day-to-day as you accelerate, that's a problem).

In addition, you will probably want clocks that run at a given perceived speed, to time physical processes that take the same time everywhere (e.g. so your cookie recipe saying "bake for 600 seconds" work without translation).