Since Google Calendar is one of the company's oldest and most ubiquitous products, you might think it would have reached this milestone already. However, it's only now joining the illustrious list of apps that have been installed from Play Store a total of one billion times.
Unlike Maps and Chrome, Calendar isn't a mandatory part of the GMS (Google Mobile Services) package, so OEMs are free to include their own calendar app instead, which many of them do. Even so, certain manufacturers whose Android skins are very close to stock still choose to go with Google Calendar, and Android One devices like those from Nokia also ship with it. It also plays a huge part in G Suite, so many will need it for work. It's not surprising that it's got to one billion, then.
What's that, you've never heard of it? You still keep track of your calendar by writing everything on the wall or in a weird little book? Come on now, get with the times and download it from the link below. At this point, I'm probably a little too dependent on it.