Apple's technical note explains this, and also tells you how you can restore reminders from a Time Machine backup. Since there is no automatic backup of reminders - they do get synced to iCloud, but there's no accessible file anywhere on OS X - this involves finding a file you've backed up and importing it, something you may not be likely to do.
It's in the File menu in the Reminders app on OS X, but I never thought of looking there, so while technically not a hint, it's good to point this out.