I would say it is probably both. calendaring tech is actually much harder than most people believe. both for usable design and efficient implementation. I say that having worked on the outlook.com/hotmail team a few years ago.

and interestingly this is the second calendaring acquisition microsoft has done, having bought jump.com back in 1999.

users, plus relationships with the various services they had integrated into the app (eventbrite, foursquare, evernote, song kick, asana, todoist.....). 100 million is a lot, but sunrise was a well thought out, well executed product