--- a/pvl/dns/serial.py	Fri Jul 05 02:32:00 2013 +0300
+++ b/pvl/dns/serial.py	Tue Sep 10 12:12:02 2013 +0300
@@ -14,31 +14,62 @@
 SERIAL_LEN = 10
 
 def format_serial (date, count) :
+    """
+        Format serial as string.
+
+        >>> format_serial(datetime.date(2013, 9, 10), 0)
+        '2013091000'
+        >>> format_serial(datetime.date(2013, 9, 10), 1)
+        '2013091001'
+    """
+
+    assert 0 <= count <= 99
+
     return SERIAL_FMT.format(date=date.strftime(DATE_FMT), count=count)
 
-def next_count (value, date, count) :
+def next_serial (date, count) :
     """
         Return serial with next count.
+
+        >>> next_serial(datetime.date(2013, 9, 10), 1)
+        '2013091002'
+        >>> next_serial(datetime.date(2013, 9, 10), 99)
+        '2013091100'
     """
 
-    count += 1
 
     # check
-    if count > 99 :
-        serial = str(value + 1)
+    if count < 99 :
+        return format_serial(date, count + 1)
+    else:
+        serial = str(int(format_serial(date, count)) + 1)
 
         log.warn("Serial count rollover: %s, %s; fallback -> %s", date, count, serial)
 
         return serial
 
-    return format_serial(date, count)
-
-def update_serial (serial) :
+def update_serial (serial, today=None) :
     """
         Parse given serial number (string), returning an updated serial, based on date.
+
+        Raises ValueError on invalid (non-numeric) serial.
+
+        >>> print update_serial('', today=datetime.date(2013, 9, 10))
+        2013091001
+        >>> print update_serial('13', today=datetime.date(2013, 9, 10))
+        14
+        >>> print update_serial('2013083501', today=datetime.date(2013, 9, 10))
+        2013083502
+        >>> print update_serial('2013083102', today=datetime.date(2013, 9, 10))
+        2013091001
+        >>> print update_serial('2013091002', today=datetime.date(2013, 9, 10))
+        2013091003
+        >>> print update_serial('2013091102', today=datetime.date(2013, 9, 10))
+        2013091103
     """
     
-    today = datetime.date.today()
+    if not today:
+        today = datetime.date.today()
 
     # handle
     if not serial :
@@ -89,17 +120,15 @@
         log.info("Updating today's count: %s, %s", date, count)
         
         # handle count rollover
-        return next_count(value, date, count)
+        return next_serial(date, count)
 
     elif date > today :
         # keep, update count
-        serial = next_count(value, date, count)
+        serial = next_serial(date, count)
 
         log.warn("Serial in future; incrementing count: %s, %s -> %s", date, count, serial)
 
         return serial
 
     else :
-        raise Exception("Invalid serial: %s:%s", old_date, old_count)
-    
-
+        raise Exception("Impossible serial: %s:%s", date, count)