http://netbsd.i2p/docs/guide/en/chap-dns.html
The serial number should be incremented
each time there is a change to the file, the usual format
is to either start with a value of "1" and increase it
for every change, or use a value of "YYYYMMDDNN" to
encode year (YYYY), month (MM), day (DD) and change
within one day (NN) in the serial number. Line 4: This is the refresh rate of the server, in this file it is
set to once every 8 hours.