§ Bouncing light clock is an hourglass

I've always disliked the "clocks" that are used in special relativity, because a clock attempts to measure something absolute, rather than something relative. So, we should rather use hour glasses. In an hour glass, we can only measure intervals of time.
Now, when we have such an hourglass, we should fill the hourglass with photons , because their speed of falling is invariant for all reference frames. So, what we need is the ability to create an hourglass worth of photons which we keep dripping down, once the photon at the funnel has reached the bottom.
This is exactly what the two mirror photon clock does --- it bounces a photon between two mirrors. We can look at this as us "flipping" the hourglass once the photon reaches the bottom of the hourglass.