Make start dates work for all modules

* error modules now hidden via access control
* get_module() returns None if user doesn't have access

common/lib/xmodule/xmodule/timeparse.py

@@ -3,9 +3,17 @@ Helper functions for handling time in the format we like.
 """
 import time
 
+TIME_FORMAT = "%Y-%m-%dT%H:%M"
+
 def parse_time(time_str):
     """
-    Takes a time string in our format ("%Y-%m-%dT%H:%M"), and returns
+    Takes a time string in TIME_FORMAT, returns
     it as a time_struct.  Raises ValueError if the string is not in the right format.
     """
-    return time.strptime(time_str, "%Y-%m-%dT%H:%M")
+    return time.strptime(time_str, TIME_FORMAT)
+
+def stringify_time(time_struct):
+    """
+    Convert a time struct to a string
+    """
+    return time.strftime(TIME_FORMAT, time_struct)