Sunrise Calendar was designed by former Foursquare engineers and won many fans for its ability to seamlessly integrate with various online services and incorporate social networking and event information from Google Calendar, Facebook, and LinkedIn.
Microsoft acquired Sunrise in February 2015 in a deal rumored to be worth around $100 million. The company subsequently announced plans in November to merge Outlook and Sunrise into a single app. That plan appears have come into effect and Sunrise will be removed from the App Store in the "next few days", says Microsoft, with the app's online backend services to cease functioning altogether on August 31, 2016.
Microsoft has gradually been introducing features to its Outlook email and calendar app, and recently added Facebook, Evernote, and Wunderlist support. Skype for Business and OneNote integration are expected to follow soon, while Touch ID support was added in March.