diff --git a/cache/cache.go b/cache/cache.go
index 863ff31..1b78b99 100644
--- a/cache/cache.go
+++ b/cache/cache.go
@@ -53,9 +53,8 @@ func (c *Cache) getFilenames(rawurl string) (hash, jsonFilename, dataFilename st
 // GetReader obtains a Reader for the specified rawurl. If a downloader
 // currently exists for the URL, a live reader is created and connected to it.
 // If the URL exists in the cache, it is read using the standard file API. If
-// not, a downloader and live reader are created. The force parameter can be
-// used to force a new download to be created.
-func (c *Cache) GetReader(rawurl string, force bool) (Reader, error) {
+// not, a downloader and live reader are created.
+func (c *Cache) GetReader(rawurl string, maxAge time.Duration) (Reader, error) {
 	hash, jsonFilename, dataFilename := c.getFilenames(rawurl)
 	c.mutex.Lock()
 	defer c.mutex.Unlock()
@@ -66,12 +65,14 @@ func (c *Cache) GetReader(rawurl string, force bool) (Reader, error) {
 			if !os.IsNotExist(err) {
 				return nil, err
 			}
-		} else if !force {
+		} else {
 			r, err := newDiskReader(jsonFilename, dataFilename)
 			if err != nil {
 				return nil, err
 			}
-			if e, _ := r.GetEntry(); e.Complete {
+			e, _ := r.GetEntry()
+			lastModified, _ := time.Parse(http.TimeFormat, e.LastModified)
+			if lastModified.Before(time.Now().Add(maxAge)) || e.Complete {
 				log.Println("[HIT]", rawurl)
 				return r, nil
 			}
diff --git a/cache/downloader.go b/cache/downloader.go
index f3c2191..a5a1824 100644
--- a/cache/downloader.go
+++ b/cache/downloader.go
@@ -6,6 +6,7 @@ import (
 	"os"
 	"strconv"
 	"sync"
+	"time"
 )
 
 // DownloadError conveys information about a download request that failed.
@@ -66,6 +67,12 @@ func newDownloader(rawurl, jsonFilename, dataFilename string) *downloader {
 			ContentType:   resp.Header.Get("Content-Type"),
 			LastModified:  resp.Header.Get("Last-Modified"),
 		}
+		if d.entry.ContentType == "" {
+			d.entry.ContentType = "application/octet-stream"
+		}
+		if d.entry.LastModified == "" {
+			d.entry.LastModified = time.Now().Format(http.TimeFormat)
+		}
 		if err = d.entry.Save(jsonFilename); err != nil {
 			d.err = err
 			return
diff --git a/server.go b/server.go
index 26bc363..e817317 100644
--- a/server.go
+++ b/server.go
@@ -11,6 +11,7 @@ import (
 	"net/url"
 	"strconv"
 	"strings"
+	"time"
 )
 
 // Server acts as an HTTP proxy, returning entries from the cache whenever
@@ -33,29 +34,21 @@ func rewrite(rawurl string) string {
 	return rawurl
 }
 
-func shouldForce(req *http.Request) bool {
+func maxAge(req *http.Request) time.Duration {
 	d, err := cacheobject.ParseRequestCacheControl(req.Header.Get("Cache-Control"))
 	if err != nil {
-		if d.MaxAge == 0 || d.NoCache {
-			return true
-		}
+		return time.Duration(d.MaxAge)
 	}
-	return false
+	return 0
 }
 
 func (s *Server) writeHeaders(w http.ResponseWriter, e *cache.Entry) {
-	if e.ContentType != "" {
-		w.Header().Set("Content-Type", e.ContentType)
-	} else {
-		w.Header().Set("Content-Type", "application/octet-stream")
-	}
 	l, err := strconv.ParseInt(e.ContentLength, 10, 64)
 	if err == nil && l >= 0 {
 		w.Header().Set("Content-Length", e.ContentLength)
 	}
-	if e.LastModified != "" {
-		w.Header().Set("Last-Modified", e.LastModified)
-	}
+	w.Header().Set("Content-Type", e.ContentType)
+	w.Header().Set("Last-Modified", e.LastModified)
 	w.WriteHeader(http.StatusOK)
 }
 
@@ -67,7 +60,7 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 		return
 	}
-	r, err := s.cache.GetReader(rewrite(req.RequestURI), shouldForce(req))
+	r, err := s.cache.GetReader(rewrite(req.RequestURI), maxAge(req))
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		log.Println("[ERR]", err)