nsupdate for serial number!

[Shanmugam Palaniappan]

Hi,
   I am running BIND 8.3.3 on solaris 8. I have tried nsupdate, the
  command line tool to make dynamic updates to the zone data files.
  It works fine but it looks like it does not change the date/month
  values in serial number.

  I have the serial number format as YYYYMMDDNN. nsupdate increments
  only the NN values but YYYYMMDD remains same.

  FYI, it works the same way with BIND 9.2.1 too.

  I would like to know whether this is what supported by BIND or this
  is how it is expected to work for some reason..
  
  Appreciate your time for your response.

Shan