Can't add events to iPhone calendar which are more than 1 year in the future
I think it's some kind of glitch in your phone or settings are not correct.

I checked on my iPhone 5, which runs iOS v.9.2.1 and event was saved even 3 years further. It's doesn't depend is it 1 year ahead, or 1 day ahead.

Try to troubleshoot:


When you are in a Calendar App, click calendars button in the bottom, and make sure the proper calendar is selected with checkboxes.
Check if the settings of synchronization for calendars are correct. Settings->Mail, Contacts, Calendars. 
Try placing event to some other, closer date, closer than one year, if it doesn't work - proceed further.
Try a Restart. Press and hold the Sleep/Wake button for a few seconds until the red "slide to power off" slider appears, and then slide the slider. Press and hold the Sleep/Wake button until the Apple logo appears.
Try system update, check if there are newer versions of iOS Settings->General->Software update.
Resetting your settings. You can also try resetting all settings. Settings->General->Reset->Reset All Settings. You will have to enter all of your device settings again.