Time is of the essence

In this context, the phrase ‘of the essence’ means extremely important. Bartlett’s Familiar Quotations maintains the saying itself is anonymous and does not pre-date 1800. Originally, it was, and still is, a legal term that appears in contracts or agreements signifying that the dates and times specified are extremely important and must be adhered to, otherwise the contract falls away. Outside of this legal usage it simply means time is critical and has been used in that sense since the early 19th century.