Code review comment for lp:~percona-toolkit-dev/percona-toolkit/fix-pt-heartbeat-2.1.8

Mixing will cause the same differences because --utc is just like using one instance on a box using UTC. So if other instances are not using --utc and are using other time zones, the same differences arise. I don't think it's worth testing this because it's always been a limitation of the tool: you have to use the same tz. --utc just makes it easy to do that now: run all instances with --utc.