diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,4 +1,8 @@
-## 1.4.16.1
+## 1.4.16
+
+* Add `guessApproot` and `guessApprootOr`
+
+## 1.4.15.1
 
 * bugfix neverExpires leaked threads
 
diff --git a/Yesod/Core.hs b/Yesod/Core.hs
--- a/Yesod/Core.hs
+++ b/Yesod/Core.hs
@@ -71,6 +71,9 @@
     , MonadWidget (..)
     , getRouteToParent
     , defaultLayoutSub
+      -- * Approot
+    , guessApproot
+    , guessApprootOr
       -- * Misc
     , yesodVersion
     , yesodRender
diff --git a/Yesod/Core/Class/Yesod.hs b/Yesod/Core/Class/Yesod.hs
--- a/Yesod/Core/Class/Yesod.hs
+++ b/Yesod/Core/Class/Yesod.hs
@@ -56,6 +56,7 @@
 import           Yesod.Core.Widget
 import Control.Monad.Trans.Class (lift)
 import Data.CaseInsensitive (CI)
+import qualified Network.Wai.Request
 
 -- | Define settings for a Yesod applications. All methods have intelligent
 -- defaults, and therefore no implementation is required.
@@ -826,3 +827,30 @@
   where
     line = show . fst . loc_start
     char = show . snd . loc_start
+
+-- | Guess the approot based on request headers. For more information, see
+-- "Network.Wai.Middleware.Approot"
+--
+-- In the case of headers being unavailable, it falls back to 'ApprootRelative'
+--
+-- Since 1.4.16
+guessApproot :: Approot site
+guessApproot = guessApprootOr ApprootRelative
+
+-- | Guess the approot based on request headers, with fall back to the
+-- specified 'AppRoot'.
+--
+-- Since 1.4.16
+guessApprootOr :: Approot site -> Approot site
+guessApprootOr fallback = ApprootRequest $ \master req ->
+    case W.requestHeaderHost req of
+        Nothing -> case fallback of
+                       ApprootRelative  -> ""
+                       ApprootStatic t  -> t
+                       ApprootMaster f  -> f master
+                       ApprootRequest f -> f master req
+        Just host ->
+            (if Network.Wai.Request.appearsSecure req
+                then "https://"
+                else "http://")
+            `T.append` TE.decodeUtf8With TEE.lenientDecode host
diff --git a/Yesod/Core/Internal/Session.hs b/Yesod/Core/Internal/Session.hs
--- a/Yesod/Core/Internal/Session.hs
+++ b/Yesod/Core/Internal/Session.hs
@@ -16,6 +16,7 @@
 import Yesod.Core.Types
 import Yesod.Core.Internal.Util
 import qualified Data.IORef as I
+import Control.AutoUpdate
 
 encodeClientSession :: CS.Key
                     -> CS.IV
@@ -44,25 +45,28 @@
 ----------------------------------------------------------------------
 
 
--- Mostly copied from Kazu's date-cache, but with modifications
--- that better suit our needs.
+-- Originally copied from Kazu's date-cache, but now using mkAutoUpdate.
 --
 -- The cached date is updated every 10s, we don't need second
 -- resolution for session expiration times.
+--
+-- The second component of the returned tuple used to be an action that
+-- killed the updater thread, but is now a no-op that's just there
+-- to preserve the type.
 
 clientSessionDateCacher ::
      NominalDiffTime -- ^ Inactive session valitity.
   -> IO (IO ClientSessionDateCache, IO ())
 clientSessionDateCacher validity = do
-    ref <- getUpdated >>= I.newIORef
-    tid <- forkIO $ forever (doUpdate ref)
-    return $! (I.readIORef ref, killThread tid)
+    getClientSessionDateCache <- mkAutoUpdate defaultUpdateSettings
+      { updateAction = getUpdated
+      , updateFreq   = 10000000 -- 10s
+      }
+
+    return $! (getClientSessionDateCache, return ())
   where
     getUpdated = do
       now <- getCurrentTime
       let expires  = validity `addUTCTime` now
           expiresS = runPut (putTime expires)
       return $! ClientSessionDateCache now expires expiresS
-    doUpdate ref = do
-      threadDelay 10000000 -- 10s
-      I.writeIORef ref =<< getUpdated
diff --git a/yesod-core.cabal b/yesod-core.cabal
--- a/yesod-core.cabal
+++ b/yesod-core.cabal
@@ -1,5 +1,5 @@
 name:            yesod-core
-version:         1.4.15.1
+version:         1.4.16
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>
@@ -25,7 +25,7 @@
     build-depends:   base                  >= 4.3      && < 5
                    , time                  >= 1.1.4
                    , wai                   >= 3.0
-                   , wai-extra             >= 3.0.5
+                   , wai-extra             >= 3.0.7
                    , bytestring            >= 0.9.1.4
                    , text                  >= 0.7
                    , template-haskell