I would strongly recommend NOT running with hybrid time turned off for one simple reason: tablet history GC will not work. Therefore when you delete or update a row the history of that data will be kept forever. Eventually you may run out of disk space. The one exception is if you drop a table, then the data for that table will be permanently removed regardless of hybrid time.
You need to configure NTP correctly. "Four NTP servers is the recommended minimum. Four servers protects against one incorrect timesource, or "falseticker". " for tips in configuring NTP. https://access.redhat.com/solutions/58025