time is of the essence

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• phrase Used to express a need for haste in action or response.
• phrase Said when something must be done immediately: When transporting organs for transplant, time is of the essence.
• phrase law When used in a contract, places the other party on notice that failure to complete a required performance by a date certain set forth therein will constitute an incurable breach.