As a general troubleshooting step, you may want to try resetting your iPhone using the instructions in the following article:
Since this issue appeared after an update, I would suggest that you restore your phone if that does not resolve the issue:
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Not only did my calender (on both iPad & iPhone) lose events & change dates, but I also lost ALL scheduling completely for the first 6 months of the year & all scheduling so far for the rest of the year (including appointments, meetings, dates, trips etc.)! It just went bonkers overnight. It was fine yesterday & today...completely crazy. In addition it is taking contact information & inserting it in the calendar. I've tried all the fixes listed in these conversations & resetting all settings but nothing has worked. When I enter something into either calendar it flashes on the calendar momentarily then disappears. WHEN will this be fixed?