User blog comment:Willbachbakal/(Champion Rework) Zilean, the Chronokeeper/@comment-24385282-20160411212547/@comment-1330314-20160411230544

That's intended. This is also with the caveat that you can only rewind one ability, instead of current double-refresh (which also scales with CDR), so while you could double-cast the same effect over and over, there are many compelling reasons for you to vary things a little, since double-casting wouldn't give you more damage unless you spread out your bombs, and Zil's abilities are all strong in different situation, and weak in others. One thing to specify, though, is that double-casting an ability puts it on cooldown again, so you can't alternate between an ability and the above passive to have 100% uptime on anything.