Uncle Jimbo's Bug Huntin' Range
On a quick read through the stellar nations (no idea if the novels deal with any of this):
Austrin-Ontis arose as a corporation on Earth and independence was thrust upon them in GW 1. Besides, they've never been exactly a well-organised or reform-minded government. A-O uses the former American States calendar.
The Borealis Republic has tried many historical calendars as philosophical fashions come and go. Currently the College of Aestheticism most strongly backs the French Revolutionary Calendar.
The Hatire have proclaimed a new calendar based on the day and year of Haven as interpreted by the words of the Cosimir.
Insight uses the Rationalised Day (see below) but assigns no work or break periods. This means that Inseers actually work longer on average than Employees, as many would rather spend time in the Grid than on enjoying meals or sleep.
The Nariac Domain evolved from the bitter struggle to colonise Naria, crystallised by the Treaty of Earth. The Nariac calendar uses Naria's day and year with 2267 (Revised Solar Time) as Year Zero.
The Orion League, with no clear historical mandate from its member nations, accepts Revised Solar Time (see below) as a compromise.
The Orlamu, another group developed on Earth, require the most precise and scientifically-accepted timekeeping and thus use Stellar Time, recalibrating Revised Solar Time based on the precisely known emissions of numerous pulsars.
The Orlamus, naturally, support this rational model in the Concord Calendar Reform Committee, and 9 of the 12 stellar nations oppose it purely because the Orlamus support it. It has been a standard widely accepted among astrophysicists, though, since shortly after the Treaty of Earth.
The Rigunmor Star Consortium's lifeblood is trade as it has been from its Earth beginnings. Rigunmors use an updated Asiatic Federation calendar internally (derived, ultimately, from the customs of the Terran Empire, before the reforms that followed the First Galactic War) but keep track of the current time of multiple trading partners.
StarMech developed, like the Nariacs, slowly from scattered colonies, but unlike the Domain their constituents were spacegoing and technologically-based. They maintain the old Microtel calendar (long since Rationalised within Voidcorp itself).
The Thuldan calendar uses Earth's day and year, as befits the bearers of humanity's destiny. Year 3000 begins on Founder's Day (2263 Revised Solar Time).
Solars use Revised Solar Time, ingeniously recalculated after the Treaty of Earth in order not to favour any one region of Earth.
The archives of Voidcorp record extensive experiments, with the self-sacrifice of many Employees, to determine the optimum productivity cycle of the human body. The Rationalised Day of 25 Earth hours has been humanely divided to provide for the biological necessities of all Employees: ablutions - one half hour, first work period - five hours, meal break - one half hour, second work period - five hours, meal break - one half hour, sleep period - four hours, third work period - five hours, meal break - one half hour, and sleep period - four hours. All secondary activities interfering with scheduled work periods must be cleared by medical leave. Each day is numbered. Years are unnecessary, though the organisation tolerates the tendency of those who will never climb further than middle management to cling to 300-day budget cycles.
The Galactic Concord has no official calendar at present, though a Calendar Reform Committee has been thrashing the issue back and forth since about six months after its establishing Treaty. Any Concord mission carries the clocks of all participating member nations. The t'sa ignore this process and use the dating systems of the Five Worlds according to their own intricate customs.
Default equipment includes "handheld electronic devices" that include timekeeping functions. In the sourcebook, the only stellar nation representative wearing a watch is the Rigunmor on page 59, who, interestingly, wears five, though those could be antique trade goods (he seems to be checking the time, though). Other possibilities are dataspecs or biowatch implants. These work by miniature atomic clocks that maintain a universal timestream - a direct record of seconds - that's then translated into a digital time and date readout by a software profile for each world or nation, so switching between calendars is merely a press of a button, or at most buying a cheap data-slip at the spaceport to load in the local time settings.