Ok, here's how it goes.
On land, time passes at 5 minutes per tick. At sea, time passes at 4 minutes per tick.
On land, maximum time acceleration is 3x, regardless of circumstance.
At sea, maximum time acceleration is 8x, unless you are in battle-mode; then the max is 6x.
You may also decelerate time; that is, make the speed slower than 1. As noted above, the way to do this is to use the keypad "+" and "-".
The "R" key will bring you back to normal speed when accelerated or decelerated. However, a second press does not return you to the previous state of acceleration or deceleration, it simply defaults to 2x.