dnssec-signzone can generate a serial number in "unixtime" format (number
of seconds since UNIX epoch).  In RT #23849 we're adding this ability to
named for DDNS updates as well. But neither one can generate serial numbers
in the human-readable date format that's in common use: YYYYMMDDNN
(where the Ys represent a four-digit year, Ms a two-digit month, Ds a
two-digit day, and Ns a two-digit intraday serial number).  That's a
feature I've wanted for a long time, and I think it would be used by
quite a few people if it were an option.

Essentially, the algorithm would be:

    if (year * 1000000 + month * 10000 + day * 100 > current_serial + 1)
        serial = year * 1000000 + month * 10000 + day * 100;
    else
        serial = current_serial + 1;

(Note: The above assumes serial-number arithmetic is used for the
comparison.)

If the zone was updated more than one hundred times in a day, then
the serial number would just have to be incremented by one; this could
result in the date as depicted in the serial number being "wrong", but
would be otherwise harmless.