"Time is of the essence" is a term in British and United States 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.
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.
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.