Google Calendar is now available as an official app on the Play Store. It's an updated version of the app that ships on many Android devices already; there aren't many big changes beyond a refreshed color scheme and a new icon. You can also pinch to zoom on individual days, there's an option to set a home time zone to make traveling less confusing, and you're now able to view events up to a year in the past.
What it does mean is that owners of devices where the calendar has been replaced by proprietary alternatives — S Planner on Samsung's TouchWiz, for example — are now able to get Google's own official version. To that point, Google does note that there may be some slight compatibility issues with some HTC phones, but this will be a welcome addition to the Play Store for many Android users. The app is free and works with Android devices on version 4.0.3 and up.