After the initial Federation-Centauri Conflict, there was a major movement to standardize for the purposes of trade and to push the Federation's agenda of cultural unity. These elements are recognized by both the Commonwealth and Federation as well as, by proxy, most other factions.
A surprisingly successful attempt at a standard language as been produced with Standard English, which is spoke by both sides when operating on national levels. This took far more negotiation and dispute than any other point on the negotiation table, though it was agreed that standardization to pre-Exodus English would be necessary as a trade pidgin as only English had a large enough speaking population on both planets.
English, like all other languages, undergone a great deal of splintering and changing and on first encounter, it is recorded that English speakers could only barely recognize each other`s tongue. Even within factions, Atlantican, Tyne and Edenic English are radically different while Muspel English might sound Danish to the ears of a Beaconer.
However, the standard written form remained relatively unchanged in both Sol and Centauri save for the introduction of new loanwords and constructions. The goal of the board was to create a standard dictionary removing most of the later additions. This was presented as a formal English necessary for trade and as something to facilitate communication between groups, it was quickly adopted by English speakers and with some grumbling slowly became the standard for all of Humanity in international travel.
This is represented through translation convention as 21st Century English, often with Centauri leaning towards British spelling and Federation towards American to represent differences in accent. There would be certain innovations developed during the two intermediate centuries which have not been accounted for and outside of the Diplomatic corps, Federation and Commonwealth citizens would still have a hard time picking through each other's accents; this is to be ignored for the convenience and ease of gameplay.
During the Exodus and Dark Age, many groups moved to establish their own calenders. For practical purposes, most groups maintain these local calenders for planetary scale business and trade as year and day length is often quite different on individual planets. Aube has moved towards a Metric time telling system for local purposes which was later adapted by resident colonists on Midgard.
Prior to encounter with the Federation; The Centauri used Boniface Calender, which dated from their arrival on the planet and used both Eden`s seasons and day length as standard, with the center of Emperor`s Estate marking the standard time for those in space.
However, the Federation insisted on the readaptation of the Gregorian Calender for interstellar trading and this measure has been largely adopted by the Centauri. Some continue to use Boniface for the purposes of internal trading, however it has otherwise been reduced to the common Nova-Centauri System Calender.
The quickest movement was the one to Metric standardization as both factions had long adopted it for space travel. Some discrepancies emerged due to the long absence of the Commonwealth, but for the most part they accepted the official measurements of the Feds. This appears to be a minor issue, however a couple early Centauri attempts at streamlining Flow travel and Federation attempts of imitating principles of the Centauri ATMCs ended in tragedy due to the minor differences in measurement systems which were not properly accounted for.
Briefly, a Galatic Credit was adopted both to operate between traders and as a standard for both the Federation Dollar and Centauri Pound. However, it was a dicey proposition from the start and when war reignited, the currency crashed.
Currently, the going exchange rate is approximately 1.71 Dollars to the Pound [0.58 FD to CCP] though this may vary by a slightly depending on the dealer as there is no official agreed trading value and both operate off of their own standards.