Tax appeal deadline
Your appeal must reach the county tax board by April 1 or 45 days after your town finishes mailing assessment notices, whichever is later. In a town that put a townwide revaluation or reassessment into effect that year, the deadline is May 1.
New Jersey example
Say your town finished mailing assessment notices on March 1. Forty-five days later is April 15, which is later than April 1, so your appeal must be received by April 15. A postmark is not enough. In Burlington, Gloucester and Monmouth counties, January 15 takes the place of April 1. A deadline that falls on a weekend or legal holiday moves to the next business day.
Official source
Form A-1 Petition of Appeal and Instructions (NJ Division of Taxation, Form A-1 (6-26))
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.