Legal Question in Family Law in California

You need to provide more information in your question. A writ of cert. would not generally HAVE an expiration date. A writ of cert. most commonly comes from the U.S. Supreme court and those most definitely would never have an expiration date, nor would one ever be postmarked late even if there was any deadline for the Clerk of the Court to mail it in the first place. The rules would be somewhat different in lower courts, but they almost never issue such a writ. In California state courts there is nothing actually called a writ of certiorari - the term in the State Constitution is "writ of review." Without identifying the issuing court and the nature of the issue that the court issued the writ for, there is no way to make sense of your question.