"Time is of the essence" is a term in contract law which indicates that the parties to the agreement must perform by the time to which the parties have agreed if a delay will cause material harm, such as the court's interpretation in the case of Foundation Development Corp. v. Loehmann’s Inc. 788 P.2d 1189 (Ariz. 1990). In that case, a minor delay was not held to cause material harm and thus no breach of contract occurred.
Compare this to an "express clause" where a specific contract term must be performed to avoid breach, such as in the court decision in Dove v. Rose Acre Farms, Inc. 434 N.E.2d 931 (Ct. App. Ind. 1982).
Contrast this with reasonable time, where a delay in performing may be justified if it is reasonably required, based upon subjective circumstances such as unexpected weather. Black's Law Dictionary (2nd Pocket ed. 2001 pg. 584).