Replace dateadd() with datediff() to get an adequate approximate (30%ish).

     select distinct SessionId from [User].Session     -- 1.2M est, 3.0M act.
      where datediff(day, CreatedUtc, sysutcdatetime()) < 365

This appears to be a bug similar to [MS Connect 630583][1].

Option recompile makes no difference.

[![enter image description here][2]][2]


  [1]: https://connect.microsoft.com/SQLServer/feedback/details/630583/incorrect-estimate-with-condition-that-includes-datediff
  [2]: https://i.sstatic.net/ApLAB.jpg