why do future calendar events get deleted

My wife has the iphone 4 (not s) and I have the Iphone 5 (not s).

They both seem to loose future dated events. We do not synch unless we are adding music or photos, the last time this was done was over 3 months ago. Also, we do not like using the icloud. We have the history to go back to ALL dates. So it is hard to understand why the phone would randomly pick an choose dates to delete.

Does anyone know why this happens or what causes this?