Timequest User Guide and Source-Synchronous Outputs

If using a clock going off chip, apply a create_generated_clock assignment to the output port. It may not be any different than the input clock, i.e. there aren't any -multiply_by/-phase/etc. attributes, but you're telling timing HOW the clock gets off chip, and then using that path for reference.

Always do a "report_timing -detail full_path" to make sure the clock analyzes back to the source where it comes in. There is a warning if the generated clock can't be traced back, but users often ignore warnings.

And yes, output clocks show up as unconstrained for me too. I let it be. You can add constraints and then cut them to get this to go away, but not worth the hassle, in my opinion.