Calendar for Windows Phone 8.1 is represented in the Store here:
Calendar can help keep your life organised with just a few taps. Add meetings and events on the fly, scroll through your schedule with new interactive views, synchronise your other calendars with ease and much more.
In addition to fixing the erroneous update, referred to here by Joe Belfiore:
WP8.1 Calendar fix is ready and getting posted to the store. Will take a bit of time to show up everywhere. Sorry for the inconvenience!
There are stability and performance improvements, though note that there are still oddities to do with 'all day' entries, so I'm sure there are other Calendar updates to come as Windows Phone 8.1 reaches maturity in the next few months.
The botched update yesterday annoyed quite a few enthusiasts around the Web, though all of these were on the 'beta' Developer Preview, so some hiccups are understandable. What's less forgiveable is that Lumia 630 owners (a device which shipped with Windows Phone 8.1 a week or two ago) were also hit with the faulty update - and these novices will have had far more right to complain at the lack of QA from Microsoft....