Last year Microsoft was planning to design a single app using Outlook and Sunrise app.But now they are trying to remove this Sunrise App.From August 31st it will be completely removed from the App Store.
Calendar services in Outlook iOS and Outlook Android apps are seen in good standards. Microsoft added Facebook,Evernote and Wunderlist into Outlook.
Sunrise supports many other calendar services and will be added to Outlook in near future.