You just stick to the current fortress timescale and stay with it. You could make full moons last longer, have week long day and nights, and it would all come toghether. As for it taking several weeks for armies to move off the map, note that it's an abstraction. What would really happen is that they prepare and stuff for several weeks, and then leave, however, this is not shown in fortress mode
That has the deficiency of being really silly. Especially in a game that models toes and eyelids.
Problem is, not only will all of these ideas involving a faster timeline take a while to implement (not a problem for something that's definitely a good idea), there's one of two problems depending on if you can switch or not. Either you take forever to go anywhere, or you have a group of issues depending on how you switch. Let's say for the sake of argument that non-switching time tweaks are a bad idea and hear how you suggest the switching work. To start, is it always player-chosen or are there some things that switch it to fast-mode or slow-mode?