I was reading this wikipedia article about contact between the Roman Empire and Ancient China, and this part about an explorer named Gan Ying jumped out at me-
While he intended to sail to the Roman Empire, he was discouraged when told that the dangerous trip could take up to two years.
How would someone from China and someone from the Parthian Empire agree on what two years was? They both would've used drastically different calendars, correct? Also that bit about it taking TWO YEARS to get to Rome from Mesopotamia is nonsense, right?
The ancients were able to come to reasonable approximations of a solar year through astronomical observation. Calendars differed mainly due to the fact that the lunar cycle is incommensurate with a single solar year and thus many different and incompatible systems of lunisolar calendars were devised. You can learn more about how classical civilizations dealt with calendars from Bickerman's Chronology of the Ancient World.
The journey from Upper Mesopotamia to Rome by land would take 131 days according to ORBIS. By open sea, it could take as little as two months.
However, discounting the possibility that there were canals at different times connecting the Nile to the Red Sea, it was impossible to reach the Mediterranean from the Indian Ocean. A quote of two years fits with Herodotus' account of the Phoenician expedition sent by Pharaoh Necho to circumnavigate Africa, which is perhaps the way Gan Ying thought he would have to go.