The serial number is an unsigned 32 bit value assigned to your SOA record must be between 1 and 4294967295.
We will issue a warning if your serial is either invalid by being outside of the allowed range or if it does not conform to this format.
Additional Information
It has become common to set your serial number with a date format to make it easier to to manage. This format uses 10 digits to represent the date and then a two digit sequence number with the format of YYYYmmddss.