I am seeing this problem on the webpage and only when entering an End Time of 12:00 am (midnight). Everything works when a different time (e.g., 1 am) is used.
I can successfully add an End Time of 12:00 am to an Event on my iPad and it will successfully sync to my iPhone. The change made on my iPad also seems to sync with the webpage. However, the End Time on the webpage changes to blank, not 12:00 am.
If I add an End Time of 12:00am on the webpage, I am allowed to save the change and it successfully syncs with my iPad and iPhone, but the End Time field returns to blank on the webpage. Hope this makes sense...