There are many ways in the ancient world of measuring the passage of time, Olympids, or years of the city (Rome anno Urbis conditae), consular years, regnal years (where there are kings) and so forth. All are tied up with the different perceptions of time, such as cyclical, linear or static.
Presumably, with advice from a group of Greek experts, Julius Caesar prompted a reform adopted by the Senate in 46 BC, which became adopted as the standard Julian calendar. It brought everywhere, in theory under one form of time, with the same number of days in the years, all good for co-orientating government, and terms of official office, across space. It also standardised the practical calendar with the tropical (equinoxes) year, vital for predicting solstices and, of course, important religious occasions. The new calendar was supposed to be always aligned with the solar year, without the need for silly adjustments. To make human-derived time work with the orbital wobble of the earth, which of course they knew nothing of – merely that the equinoxes could occur at different but regular times, leap years of 366 days were introduced to cover this. The problem was that they miscalculated the use of leap years, and so still found time going out of sync.
In 1582, Pope Gregory XIII announced a new reform. This made the year slightly shorter, but more importantly got the leap years correct in finally bringing time into harmony with the equinoxes and solar time. Every year (of the Christian calendar) divisible by four is a leap year, which underscores the importance of having a unified calendar. In the Islamic world, they have a different solution. For example, some places such as Ethiopia still use Julius’ calendar as does the Greek Orthodox Church, I think, which is why their main festivals are out of sync with western agreed ones. Lots of non-western cultures have adopted its calendar as CE (common Era) in order to be able to co-ordinate globalised life across the world.
Huge experimentation was done in the Renaissance to study the actual movement of time in various places. There are still famous meridian lines on the floors of churches and cathedrals in Roma and Palermo, where the crossing of the line by the sun could be measured, thus harmonising time in widely separated places.
Anyway, that’s why we have leap years, and why time is not a fixed constant as well as the origin of time zones, for example. The debate continues, of course, and we now also have quantum times along with the idea of the multiverse as mainstream physics. It seems that observation is vital, and observation of events in the past can affect their manifestation in the present (If I understand it), leading to the whole possible of quantum waveforms and the divergence of realities, which also provides a mathematical possibility of time travel at a quantum level of course. Believe that if you will!
An opinion piece, by Oliver Gilkes.