Bash. By default it might seem less featureful than zsh… but bash is a lot more powerful and extensible than some give it credit for. It might be more complex to set it up the way you like it, but once you do it, that configuration can be ported over wherever bash exists (ie. almost everywhere).
Note that the Sun position is not consistent throught the year and varies widely based on your latitude.
In Iceland (and also Alaska) you can have the Sun for a full 24 hours in the sky (they call it “midnight sun”) during Summer solstice (with extremelly short nights the whole summer) and the opposite happens in Winter, with long periods of night time.
Just the days of the week? you mean that 2024-06-30 23:59 and 2024-07-01 00:01 can both be the same weekday and at the same time be different days? Would the definition of “day” be different based on whether you are talking about “day of the week” vs “universal day”?