diff --git a/lib/kgrader/backend/svn.rb b/lib/kgrader/backend/svn.rb
index 7abd407..3f9a797 100644
--- a/lib/kgrader/backend/svn.rb
+++ b/lib/kgrader/backend/svn.rb
@@ -26,6 +26,18 @@ module KGrader::Backend
       run *args, repo
     end
 
+    def log(repo)
+      xml = Nokogiri::XML run('log', '--xml', repo).first
+      xml.css('logentry').map do |elem|
+        { :rev  => elem.attr('revision').to_i,
+          :date => Time.parse(elem.css('date').text) }
+      end
+    end
+
+    def commit(repo, message, paths = nil)
+      # TODO
+    end
+
     private
     def run(*cmd)
       Open3.capture2e('svn', *cmd)
diff --git a/lib/kgrader/errors.rb b/lib/kgrader/errors.rb
index 34afe05..568826e 100644
--- a/lib/kgrader/errors.rb
+++ b/lib/kgrader/errors.rb
@@ -14,9 +14,15 @@ module KGrader
   class CourseError < KGraderError
   end
 
+  class RosterError < KGraderError
+  end
+
   class AssignmentError < KGraderError
   end
 
-  class RosterError < KGraderError
+  class TaskError < KGraderError
+  end
+
+  class SubmissionError < KGraderError
   end
 end
diff --git a/lib/kgrader/submission.rb b/lib/kgrader/submission.rb
index ecbfa51..c2f98ac 100644
--- a/lib/kgrader/submission.rb
+++ b/lib/kgrader/submission.rb
@@ -41,8 +41,8 @@ module KGrader
         oldrev = revision if status == :graded
         self.status = :fetching
         @course.backend.update repo
-        rewind due
-        self.status = revision == oldrev ? :graded : :ungraded
+        newrev = rewind due
+        self.status = newrev == oldrev ? :graded : :ungraded
       end
       nil
     end
@@ -63,7 +63,8 @@ module KGrader
     def commit
       # TODO:
       # if status == :graded && File.exists? pendingfile
-      #   [copy gradefile to repo and commit]
+      #   [copy gradefile to repo]
+      #   @course.backend.commit repo, <message>, <gradefile path>
       #   FileUtils.rm pendingfile
       # end
 
@@ -93,7 +94,15 @@ module KGrader
     end
 
     def rewind(date)
-      # TODO
+      log = @course.backend.log repo
+      target = log.find { |commit| commit[:date] <= date }
+      if target.nil?
+        raise SubmissionError, "no commits before due date: #{student}"
+      end
+
+      rev = target[:rev]
+      @course.backend.update repo, rev
+      rev
     end
   end
 end
diff --git a/lib/kgrader/task.rb b/lib/kgrader/task.rb
index 7ede464..4ebf7e0 100644
--- a/lib/kgrader/task.rb
+++ b/lib/kgrader/task.rb
@@ -16,6 +16,10 @@ module KGrader
       fetch = options.fetch(:fetch, true)
       regrade = options.fetch(:regrade, false)
 
+      if options.include?(:due) && !fetch
+        raise TaskError, "can't set a new due date without fetching"
+      end
+
       subtask 'setup' do |sub|
         sub.create unless sub.exists?
       end