A 2001 CNet article by Sergio G. Non:
For years, publicly traded companies have been citing rules from
the U.S. Securities and Exchange Commission as reasons not to
discuss almost anything related to corporate operations in the
weeks preceding a quarterly report. “SEC-mandated quiet period”
often becomes a boilerplate phrase for public-relations personnel
when earnings are less than a month away.
Yet it’s not even an SEC rule.
“The quiet period phenomenon is a matter of practice rather than
regulation,” SEC spokesman John Heine said. “You’re not going to
find a regulation that says ‘you have to be quiet for 30 days’ or
something like that. However, there are provisions in securities
laws that lead people… to be careful in how they handle certain
types of information.”