Well, the day problem is a complex matter. If you have longer, more "realistic days", assuming nothing else change in the remaining features...
... Long jobs are done overnight.
... Merchants take ages to arrive.
... The fortress will be mature before the end of the first year.
... You'll have to wait a long time to see all the "seasonal features" freeing, evaporating, colored trees etc.
But if you have short days, like the one we're having....
... The eat / drink / sleep cycle is totally broken. Dwarves sleep half a month or take a week to eat their lunch (exaggerating a bit).
... Wounded dwarves can take years to recover from a broken arm.
... Or days to travel from a part of the map to another.
We can't fix that easily.
Sure you can. Time is already measures in ticks (see crop time), so may as well use that to make the most of it.
Areas where you'd need customisable times (in ticks) to make this moddable:
Crop growth duration
Day length (solves issue of how little a dwarf is able to do in a day, especially movement-wise)
Base Task length (solves issue of long jobs getting completed overnight)
Naturally, this all drastically impacts the gameplay in the following ways:
Fortress building takes a lot longer
Fortress has a lot less clutter.
If you make dwarves eat more often, then you'll need bigger crops and bigger food storage areas.
Merchants will seem rarer, and will have a greater impact on the resources available to the fortress.
The biggest problem: More time where a player has nothing to do. Of course, this will change as there becomes more things to do in fortress mode, and the dwarfs starting having more to do in a day.