On one machine, time is easy. Call now(), get a timestamp, and if event A's timestamp is smaller than event B's, A happened first. Sort by created_at and you have the true order of things.
Across many machines, that stops being true. Each machine has its own clock, and no two clocks agree exactly. Usually they're off by a few milliseconds, sometimes by seconds, and occasionally one jumps backwards. If your design quietly assumes timestamps from different machines can be compared, it will eventually put events in the wrong order and lose data while doing it.
Every machine has its own clock
A server keeps time with a small quartz crystal that vibrates at a known frequency. "Known" is generous. Crystals vary from chip to chip and drift with temperature. A typical one is accurate to somewhere around 20 to 50 parts per million.
That sounds tiny, so do the maths:
50 parts per million × 86,400 seconds in a day ≈ 4.3 seconds a day
Left alone, two Tiffin servers could disagree by several seconds within a day, and by a minute within a couple of weeks.