[Bro] Howto

[Vern Paxson]

> 	if ( rec$not_valid_before > network_time() - 60*60*24*30 )

Note, this will need to be:

	if ( rec$not_valid_before > network_time() - 60*60*24*30 sec )

Bro won't allow mixing integers with time values.

		Vern