How to delete all calendar events from an iPhone 4 with iOS 7.1.2?

Do you sync calendars from another device or the Internet (e.g., Google calendar, Outlook, etc). If so, just delete them on the other platform and sync the phone. If the iPhone is your only calendar and you don't sync it with anything you will have to do it the slow way. IIFC there is a setting on the 4 to delete calendar entries older that a period of time (like 1 month). Check in Settings/Mail, contacts, calendars.