Confused with set_input/output_delay command

It's not normally negative, but with clock skew it's possible. I think I was trying to show a larger range between -max and -min, and wasn't thinking about how often a set_input_delay -min is actually negative.

I didn't explain the data-clock thing very well. I've actually written two different write-ups and realized they weren't going anywhere and probably not much different than what's in the User Guide. This can get difficult to talk about in the abstract and might be easier with what you're actually trying to do. Can you give an example of an interface you're trying to constrain, what you're trying to do and what you're confused on? I think that might be a better way to figure this out.