The first thing I tend to think of is using pulsars, as described above, and the thought that follows that for me is exactly as you describe, to watch the flashes of light emitting from the angien egg and count them to be able to use them as our time keeping method. Something that theoretically should be visible at nearly the same intervals for everyone observing (considering flash player isn't some overarching factor denying characters from being in the same scene-universe and synchronizing the light rays, along with similar technical issues) and is something that is constant so our "clock" doesn't fail us a day later. From my observations there is 5 flashes of light before the cycle continues, so I posit we only need to count those 5 for an accurate measurement (feel like I might be wrong on that though).
The main problem comes in measuring these flashes precisely as you need something else as a frame of reference to be able to determine exactly how much 'time' (" d/V. Basically, it's about movement over a distance") has passed since we began measuring the flashes of light. My initial idea was to use my pack of playing cards as my character has an uncanny ability to draw a card every 2 seconds. Unfortunately as the player I am unable to reliably refresh my inventory quick enough and redraw to be sure that I am continuing to draw cards at a steady rate. So, second idea was to use beer ???? as it has a short 60 second cooldown but another problem arises, the refresh that occurs prevents me from being able to count from start of consumption until the point that I would have another beer ready for an accurate measurement. Ok so third idea is to use a water bucket, and this one would work.
I don't feel like counting those pulses for ten minutes though and then recounting, which would be part of the 'bootstrap' method for having a reliable result and refining the measurements. Anyways if I wanted to throw out a number , say I counted 480 pulses in that ten minutes, and the cycle is 5 pulses to a single time unit we are trying to find, you get 96 units of time within that 10 minutes, considering we know what a minute is somewhat independently as we know it takes me ten of them to be able to collect a full bucket of water, 0.16 seconds per cycle of 5 pulses (I hope I'm getting that right now, time maths bother me) That seems wrong to me and certainly because I pulled the number at random, but you can divide/multiply each 'time unit' as I'm describing them to be longer or shorter, and then use that to determine how long another process takes as long as you redo the measurement. Meaning to then know that it takes me a minute to finish a beer, we can count out the time units in relation to those 60 seconds IE count 9.6 cycles of light pulses within that one minute it took to finish the beer, if the figures line up, you have a time keeping method. Of course it's not so simple considering we already know what a minute is but you cross reference measurements to make sure you can arrive at an accurate one.
A bit less science-y, I came into the topic with the idea time is between two players. Why not one? Who is to say they were ever there anyways, besides them, and they'd have to tell you.