diff -r ebab64f600c6 src/org/python/modules/time/Time.java
--- a/src/org/python/modules/time/Time.java	Sat Mar 23 18:23:02 2013 -0700
+++ b/src/org/python/modules/time/Time.java	Sat Mar 23 19:29:57 2013 -0700
@@ -764,6 +764,8 @@
         StringBuilder builder = new StringBuilder();
         boolean directive = false;
         boolean inQuote = false;
+        boolean containsYear = false;
+        boolean containsMonth = false;
 
         if (format.length() == 0) {
             return null;
@@ -802,9 +804,29 @@
             if (translated == null && notSupported.contains(charAt)) {
                 return null;
             }
+
+            switch (charAt) {
+                case 'c':
+                case 'x':
+                    containsMonth = containsYear = true;
+                    break;
+                case 'y':
+                case 'Y':
+                    containsYear = true;
+                    break;
+                case 'b':
+                case 'B':
+                case 'm':
+                    containsMonth = true;
+                    break;
+            }
+
             builder.append(translated != null ? translated : charAt);
             directive = false;
         }
+        if (containsMonth && !containsYear) {
+            return null;
+        }
         if (inQuote) {
             builder.append("'");
         }