I had an idea for a web app that would make use of a user's playtime history of all games over the last year. This seems like data Steam tracks given the Steam "Year in Review" has a "Playtime by month" section. That said, I can't find anyway to do this with the little I've read about their API. Thanks!
Unfortunately that doesn't seem to be available. You can however track playtime over the last 2 weeks and scrape it as they use your tool to keep track of it.
https://developer.valvesoftware.com/wiki/Steam_Web_API#GetRecentlyPlayedGames_.28v0001.29