I had the same problem. So I wrote a small open source app for myself, as a temporary workaround.
It's in the iOS AppStore now, for free:
Please read the AppStore app description before using it!
In the current form it simply "copies" calendar entries from iOS to OS X, deleting calendar entries on OS X first.
Thus, be sure to add and update your calendar entries on iOS only!
But still, this works well enough for me.
Three days ago, I pushed version 1.1 to the iOS AppStore, should be available sometime next week. It makes the "sync" time range configurable.
As time permits, I will add support for contact synchronization and improve merging behavior of calendar entries.
You have to get the corresponding OS X app from https://github.com/yep/DeviceSync-for-OS-X/releases
Please open the OS X app by holding the control (ctrl) key and then double-clicking the icon. For more information, see https://support.apple.com/kb/HT5290.
This app is open source and hence a collaborative effort. If you are a developer or know some iOS developers, feel free to add missing functionality (send a pull request at github).
See https://github.com/yep/DeviceSync-for-OS-X and https://github.com/yep/DeviceSync-for-iOS
I have an old iPhone 3GS and it is running iOS 4.1.
Now iOS 4.1 is a problem: it does not support ARC. I think it should be possible to update to 6.1 on the 3gs. It would be a lot easier to port this app to 6.1 than to 4.1