Can you summarize this article from the Orion's Arm Worldbuilding Project?

"Calendars and Timekeeping

At the start of the Information Age there were a number of calendars in use on Old Earth, but the one most commonly used for commerce and science was the Common Era calendar (c.e.), based on the ancient Gregorian Christian calendar. This calendar was used in conjunction with the Coordinated Universal Time standard (UTC) to give a single value for timekeeping all over the planet. UTC was adopted throughout the Solar System at first, but once permanent colonies were established on the Moon and Mars new calendars were established for those worlds. The Martian calendar was adopted in 2185 c.e and is based around the 'sol' or Martian Day, having a start date of 14 November 2031 when the first manned landing occurred. The Lunar calendar known as the 'After Tranquility Calendar' (a.t. or A.T.) was established soon after, and takes the first manned landing on the Moon in 1969 c.e. as its starting date. However the Lunar day was considered too long to be useful, so the new calendar retained UTC to count the time of the new colony. By 391 a.t. the majority of the Solar System (apart from Earth and Mars) was using the a.t./UTC standard.

After the Great Expulsion several billion refugees from Earth crowded on to the Moon and onto habitats in Cislunar space, waiting for a chance to escape to the roomier colonies of Mars and the Outer Solar System or onto the great Arkships that were being built to carry them to the nearest stars. During this time the Lunar calendar became the common measurement of time for most of the former inhabitants of Earth. The refugees almost always took the 'After Tranquility' calendar with them to their new homes among the stars. Later Luna became the capital of the First Federation in the early years of its formation, and attempted to regularize the measurement of time on an interstellar scale; this of course proved to be impossible.

In a universe governed by special relativity it is impossible to say in an absolute sense whether two events occur at the same time if they are separated in space. Time dilation changes the rate at which time passes for any moving object when observed from another moving object; so no common frame of reference can be found to measure the passing of time. On the surface of a planet, or even within a single solar system, it is possible to disregard simultaneity effects for almost all purposes; but once the Terragen Expansion spread from Sol to other planetary systems these effects became very much more important.

Interstellar spacecraft and their crews traveling at an appreciable fraction of the speed of light will experience time dilation to a greater or lesser extent according to their velocity; a journey of 6 light years at 0.6 c will take ten years as seen from an observer on the world it left from, but will only take 8 years from the point of view of the crew. Once the ship arrives, the colonists have a number of choices if they want to establish an accurate calendar. They can keep the calendar of the original world, and if they left that world in the year 1000 a.t. for instance they would count a date of 1008 a.t. as their date of arrival. However when they compare notes with the original world (a process which would involve an exchange of messages at light speed, and which would take twelve years) they would find that ten years had passed on the home world, and the calendars would be out of sync.

What is worse, the two stars concerned would be so far apart and moving so fast with respect to each other (because of their so-called proper motion) that there is no way to coordinate the rate at which their respective measures of time pass; the two calendars are essentially independent from each other and can never be reconciled. The proper motion of one star with respect to another changes constantly because of the gravitational effects of the rest of the universe, so the effects of time dilation on the measurement of time on each world when compared to the other cannot be measured accurately.

So when the First Federation attempted to standardise the After Tranquility calendar across interstellar space they found it was literally impossible. By the end of that period the Terragen Sphere was several hundred light years in radius, and the uncertainty in the calendar date from one side of the volume to the other was a significant factor.

When wormhole communication began to spread throughout this expanding sphere there was the chance to establish a new standard. A signal could be sent from any particular world to the centre of the Wormhole Nexus, which has a basically tree-like structure to this day, and is centred upon Aksijaha (also known as Vega) in the Inner Sphere. This signal could be datestamped at the centre of the Nexus and returned to the originating world, and the date at that world could be established as being the time on the datestamp plus half the traveling time to and from the centre.

The cultures of the Integration were particularly keen on establishing a common time in this way, and they continued to use the After Tranquility standard. Following on from the Integration, the Sephirotic Empires generally use the a.t. calendar to record significant events, and this is the calendar used at the department of History in Verlibcon Arcology on Ken Ferjik for the Encyclopaedia Galactica.

On the other hand, most worlds and habitats and even separate countries or polities within those worlds maintain their own separate calendars, often based on the orbital characteristics of their world and or the date of the foundation of the colony or other momentous events. Mars still uses its original calendar, and the few inhabitants of Earth still use the Gregorian measure; the oldest calendars in use are those of the alien xenosophonts on To'ul'h and Muuhhome, which date back to long before human civilization.

However the value of Common time is subject to many effects of simultaneity which makes it an arbitrary measure of time; wormhole linelayers move at considerable fractions of the speed of light, and the ends of the hole can therefore have quite different local time values on arrival. This effect can be modified by the judicious use of gravitational fields at one, or both ends. Also the passage of information through a wormhole can happen at different rates according to exact geometry of the hole. A message from a distant world may need to pass through dozens, or even hundreds, of wormholes, and the transit time can vary considerably over time. So for the outlying worlds Common time, or Empire time as it is sometimes known, is a very variable measure of time, and in no way establishes a single frame of reference for the whole Orion's Arm Civilisation.

In any case there are countless worlds in the Outer Volumes and elsewhere which are not linked to the Nexus, so are subject to the same relativistic uncertainties that plagued the First Federation. As the Nexus expands to link with new systems, very often the calendars of the new worlds are very much out of sync with Common Time. For instance the Refugium Empire was almost sixty years adrift when it was finally rediscovered, despite having consistently and accurately used the old Lunar Calendar (although without any attempt at adjusting for relativity). And relativistic polities such as the Deeper Covenant which ply the space between the Sephirotic worlds are accustomed to recalibrating their calendars on arrival, while maintaining accurate ship time as the most important and relevant measure."