Wiktionary
time killer
n. An activity which unproductively consumes a large amount of time; a distraction.
Usage examples of "time killer".
The up-time killer had reached a three-story, eighteenth-century structure that might have once been a grand house, built of grey-painted stone and mellow brick.
Then it turned out that the Toaster wasn't just a little-time killer, he's now a serial killer.