One of my favorite things about how Android handles apps is the streamlined process of updating them through the Play Store. Rather than waiting for a full system update just to fix a small bug in an app, phone-makers can push fixes through the Play Store as soon as the software is ready. Following its file browser, launcher, and many other apps, Xiaomi's Calendar is now available on the Store as well.
Version 18.104.22.168 is the first build of the app to appear on the Play Store and should be available for Xiaomi devices now. Unlike its file browser, this release is exclusively for Xiaomi phones, so if you're using something else, you won't be able to download it from Google Play. If for some reason this is still showing as unavailable for you on your Xiaomi device, then we have you covered at APK Mirror.
There doesn't seem to be anything new in this update other than the Store listing, but now that it's separate from system updates, I'd expect to see more frequent changes in the future.