Of days and weeks.
If you're using a copy of Earth's calendar, how do you justify any similarities (if you do, in the first place)?
For readers: do you question uses of things like "January" and "Saturday" when set in a world with no relation to Earth whatsoever?
I also do not like it when an author uses our names for things Monday Tuesday January February etc.
Seems like a problem no?
Personally I think the best feeling most comfortable answer to that is just the Author's 'ole reliable: Don't talk about it and let the readers brain fill it in and do the work for you.
That meaning: Don't mention if weeks or hours are different to our hours or weeks and don't use particular names of months or weeks at all, just specify the period of time if a character needs to state a time "Three weeks from now" "at the end of the week" It keeps it extremely generic and malleable feeling, like its just being translated. Like how feet and inches feels right in fantasy land, it just does. 🤷♀️
I rather aggravatingly made it into 30 hour days, which on its own causes a headache. Then there are 12 months, each with 28 days. Weeks are 7 days because it makes sense from there. The only similarity I forced from Earth was the convention of 60 minutes in an hour, and the same for seconds. I haven't actually given any explicity dates yet and normally just give time changes like 'tomorrow', or 'a week later'.
There is a plot reason for the 60 minutes/seconds thing, but my recommendation is never change the hours in a day, nor minutes or seconds. Days in a year is perfectly fine, same for their arrangement in weeks/months. But adapting to entirely new measurement systems is hellish. All the non-isekai + non-Earth stories I've read just used our time system and never went into detail of its origin. Pretty sure most traditional fantasy books do that as well.
It's a seven-day system. With four seasons and 12 months thought up as moons. It's a world similar to Earth. However, the naming is different. I've just not thought of the names yet in the full calendar. Mostly because it's a little irrelevant. I named some of the months that are important to characters. And the reason why I did it that way is to just make it easier on myself. 7 days a week and 12 months a year is easier for me to keep up with when this project stretches over a long span of time, 50+ years.
The calendar that the POV characters use, is based on the goddess Diva and her demi gods. Each month of the calendar is either named after them and if a hard reference to them. Since they are worshipers of Dias, another god, there is a slight modification in the names, but it's still a twelve-month lunar calendar.
Days aren't really names. Days are just numbered. So for example. The month of Hearth is the 4th month. Any sort of dating, would be. 1st day of Hearth + year.
Months are 30 days as that's how I've been calculating things.
I do appreciate it if an author takes the time to figure out and makes mention of it in a normal fashion with their characters! It's definitely extra points that'll help world building without being lore drop heavy.
You can usually shoe-horn an equivalent system into any setting to make it seem familiar and logical, whilst being just illogical enough to seem human.
I've not done this, but I have written stories set on Mars, accommodating a (slightly) different-length day, longer years and no seasons to speak of (as Mars has very little axial variation).
While the length of weeks and months may differ between ethnicities that tracks and names time differently, I always make a point of making sure the years are as long as they are on Earth, because otherwise it'll become a right-o hassle to track how old the characters are. I've seen a novel or two where the characters are actually a few years older than they're written to be, because the years of that world are longer than Earths. It is a bit confusing as a reader, to have to calculate how old a character is in order to get their correct age in Earth years.
Since it's set in a mostly fictionalized Orion Arm, I think there are inconsistencies regarding time disparity between NotEarth and RandomBackWater planet, including things like 7 days of the week and stuff but... I handwave it away and don't think too much about it.
The months have names roughly corresponding to the season. The Month of Rain, The Month of Embers, et cetera.
How long are they? Who fuckin' cares? An adventurer isn't going to be counting days on a calendar.
"Ah yes, today is a fine day indeed, the 24th day of the 3rd month of the 200th year!" To give an example, but I try to keep it out of dialogue entirely. This is just to help keep track of how much time has passed rather than it playing a part in the actual story. In terms of calendar days it's just 30 days a month with 12 months a year but it's never going to be explicitly stated or explained why it's like that because, again, it doesn't play that big of a role except to keep track of time.
One thing I really honestly want to see is an isekai where there's a different length day so we can get some good Non-24 sleep disorder representation, not that you need a magic portal premise for that but it would be really cool to see.
I am definitely against a fantasy or non Earth connected story using our words. Especially since talking about the calendar, the names of days and months, can be such an exellent way to slip in cultural details without infodumping.