I actually have several calanders in my world.
The foundation is a seven day week with the days inherited from the Old Faith. The week begins with Truceday, then proceeds through the elements Windsday, Earthsday, Fireday and Waterday, and lastly, Sunday and Moonday. If you don’t see the joke… say them out loud.
The humans use a 12 month calander with the months names for the 12 gods of the primary pantheon; the Via Praetorum. The months coinciding with the spring and autumn equinoxes and the summer and winter solstices have 5 weeks (35 days) while the others have 4 weeks (28 days). Each season begins with the associated solar event which is also the start of the five week month.
The Old Faith marks the seasons, but counts only the weeks instead of months (i.e Windsday of the third week of spring).
The elves, by contrast, use an entirely lunar calander (their All Mother deity is associated with the Moon) with each month beginning on a New Moon and beginning a two week cycle of inward contemplation of one of their eleven deities, followed by two weeks beginning at the Full Moon where that deity is outwardly celebrated.
The final two months of the annual cycle celebrate the relationship of two of the ten lesser gods with the All Mother (one per month) and which gods and their order give the name to that particular year and a complete cycle of those forms a 90 year cycle which themselves are named for one of their gods and is believed to reflect the fortunes of the elves during that age.
There are also several different counts of years; the Old Faith dates from The Covenant that is foundational to their faith. By their count it is Covanent Year (CY) 5204. The Praetorian Empire dated things from its founding and by that reckoning it is Imperial Year (IY) 975. Those who fell to complete barbarism count from The Cataclysm, with a date of 198 Post Cataclysm (PC). The elves use a similar count since they only entered the Mortal World because of the Cataclysm, but with their dating system it is recorded as the Year of Songs of the Seas in the Cycle of Cha-Chata (Fate).
Money is almost entirely by actual weight of the metal and so the Pound (of silver), Cent (one per cent of a pound of silver) and Bit (a quarter of a cent) are the most common for interstate trade, though each realm generally mints its own coinage in varying denominations using various metals.
Bits are typically copper and about the size of a US penny. 1, 5 and 10 cent coins are typically struck in silver (the 1 cent coin is typically about the size of a US nickel). 20 or 25, 50 cent and one pound coins are typically struck in gold, with the £1 being a 1 oz (1/16 variety, not the 1/12 Troy oz) coin about the size of a quarter (thus the de facto ratio is 16:1 for silver to gold value… which is reasonably close to historic norms in the real world).
Anything beyond about £80 (5 pounds of gold) is most likely going to be in the form of precious gems instead (which can hit up to £10 per caret fof very rare varieties) with some extremely wealthy realms striking silver or gold coins that are inlaid with one or more less rare gems for use as £5, £10 or larger values (not that there is much of anything beyond ships, artwork and magic that would require such coinage).
Measurements are generally anthropic (based on the human body). The inch was some past ruler’s finger digit, the foot is similarly someone’s foot, the yard is the length of their belt. The most common game measurement is the Pace, for which I use the Roman pacum because it is almost exactly 5’ and thus measures D&D-style 5’ squares in a very natural unit… i.e. 6 paces instead of six 5’ squares.
My in-game Mile is similarly based off the Roman mille passum (1000 paces) and so is 5000’.
Weight is based on the pound, derived from the weight of a pint of fresh water (i.e. “a pint’s a pound the whole world round”). The pint is derived from a container that comfortably fit in some past ruler’s hand (a cylinder about 3” across and 4” high). The ton is 2000 pounds and was based on the largest barrel of alcohol (250 gallons) that could be easily rolled through some past ruler’s feast hall door. Finer grades of weight are the oz (1/16 of a pound; primarily a customary unit), the cent (used for coinage and science), the carat (1/10 of a cent; 1/1000 of a pound) and the grain (1/100 of a cent; 1/10,000 of a pound) with fine weights typically recorded as X.YY.ZZ (pounds.cents.grains) with any final zeros dropped (so 4.5 instead of 4.50.00).
In practical game terms only the pound is typically relevant, though cents and carats are common with coinage and gems/jewelry.
It is entirely possible I’ve thought about this way too much.