[phyzome]

"Dear tahw, what time and date is it over there? Please write back. I shall write again next month. Sincerely, phyzome"

...

"Dear tahw, over the past year, I have noticed something interesting in our correspondences -- more hours have passed on your end than mine, on average. I believe our clocks are ticking at different rate. Yours, phyzome"

(except measuring number of ticks, as communicated over fiber optics, and possibly with other tricks to cancel out communication lag)