Darius ult CD starts once the spell goes through, even with resets. This means if you get a kill with his ult, then another reset 15 seconds later, then wait for the ult reset period to be over of 20 seconds, 35 seconds will have passed on his ult CD.
Say it was 120 then the situation said above happens, once you can't recast his ult will be on a 85 second cooldown.
Pykes doesn't work like that. It doesn't matter how many resets you get or how long you're still able to recast his ult, the CD won't start until you can't cast it anymore. If it has a 120 second CD and you get a reset every 15 seconds 3 times for a total of 45 seconds THEN wait for the 20 seconds to expire, his ult will be on a 100 second CD.
Is this a bug or intended?