I was only changing the Pitch (standard Sun rotation). I set it to print string each tick. As you would expect there were tiny increments in the angle as it looped, but as it hit -90 or below it stops at -90.0 (I had reversed direct at this point, but still stops at -90 in the same vein). I even tried to manually flip it, but that means using Set Actor Rotation, which defats the object (might as well just use that instead from the get-go) which is what I did.
Simple test, just follow the WIKI Day Night example (that uses Actor Local Rotation) as it will stop at midday - print string shows it to stop at -90.0. I may have got the number the wrong way around in the Op (270 to -90) btw. :o