Having this problem myself - seems to be under several discussion threads with slightly different variations.

Common theme seems to be Outlook.com is fine, showing all entries no matter how old;, but calendar app shows only the past few months.

Just to be thorough I performed the test you mentioned: added an appointment in Outlook.com; it showed up just fine in the app. The reverse also works fine: added an appointment in calendar app and it shows up immediately in Outlook.com. Problem is all older entries simply do not appear in the app.

I should mention I'm seeing the exact same problem with phone calendar - nothing more than a few months ago. There too, test entries show up just fine either way - it's the older ones that are simply not there.

This is embarrassing for Microsoft that something so very basic does not work as it should.

Would suggest you to follow the methods given below and check if it helps.

Run apps troubleshooter.

Follow the steps provided in the link given below to run Apps troubleshooter.

What to do if you have problems with an app
http://windows.microsoft.com/en-us/windows-8/what-troubleshoot-problems-app
Also refer to the link: http://windows.microsoft.com/en-in/windows-8/calendar-app-faq