How can I stop my previous calendar entries disappearing from my Nokia Lumia 520 phone?

I have been using the calendar on my phone only (Windows 8.1), and do not use the calendar in my Outlook account on my PC. I'm assuming the phone is syncing with this Outlook calendar which is empty. I only want to sync my email (via my Optusnet account) on my phone and NOT the calendar as it's more convenient to use the phone calendar.
I can't find any way of doing this.
I've seen suggestions for turning off calendar deletions via Outlook.com, and although I have an Outlook.com account I don't use it.
Any suggestions for fixing this so I can keep the calendar history on my phone?

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