Cron sierra

On macOS High Sierra (and perhaps earlier), cron is disabled in some configurations. There is, however, still a LaunchDaemon for it.

Check to see if cron is enabled:

sudo launchctl list | grep cron
You should see com.vix.cron.plist if cron is running. If cron is not running, you should do:

sudo launchctl load -w /System/Library/LaunchDaemons/com.vix.cron.plist
That will start cron, and the -w switch will make sure it starts after reboots as well.