My "Order Date" column is a formatting nightmare. How do I force these into one consistent format?

Hey everyone, I’ve inherited a spreadsheet (screenshot attached) where the date columns are a complete mess. It looks like a mix of different regions and manual entries. The Issues: Some are written out (11 August 2016). Some use dashes (11-08-2016). Some use slashes (6/16/2016). Worst of all, I suspect some are being read as Day/Month while others are Month/Day, making it impossible to sort them chronologically. I’ve tried highlighting the column and changing the "Number Format" to Date, but half of the cells don't change at all (which I assume means Excel thinks they are "Text"). What is the fastest way to "reset" this entire column so every date follows the same format (YYYY-MM-DD)? I'm using [Excel / Google Sheets]. Thanks in advance for saving my sanity!