
254 S.E.2d 44 (1979)
40 N.C. App. 681
Barbara MEDDERS (Davis)
v.
Ronald R. MEDDERS.
No. 7810SC491.
Court of Appeals of North Carolina.
April 17, 1979.
*46 Cheshire, Bruckel & Swann by William J. Bruckel, Jr., and Michael A. Swann, Raleigh, for plaintiff-appellant.
Barringer & Howard by Robert E. Howard, Raleigh, for defendant-appellant.
ERWIN, Judge.

Defendant's Appeal
The validity, effect, and construction of the agreement before us is governed by the law of South Carolina. 24 Am.Jur.2d, Divorce and Separation, § 884, p. 1004.
*47 Under the law of South Carolina, it is the court's duty to effect the intention of the parties in construing a contract to the end that justice may be done. Rainwater v. Hobeika, 208 S.C. 433, 38 S.E.2d 495 (1946).
Defendant contends the trial court erred in finding Section 9 of the agreement valid and enforceable. We find no error.
The provision in question provided for the payment of $700.00 monthly, commencing on the date of the agreement. It is certain as to the amount and as to the time of payment. Its only uncertainty is to the duration of performance, but this uncertainty is not fatal. Where parties to a contract express no period for its duration and no definite time can be implied from the circumstances surrounding them, the contract extends for a reasonable time. Childs v. City of Columbia, 87 S.C. 566, 70 S.E. 296 (1911). Conversely, it follows that if a definite time can be implied from the circumstances surrounding the execution of the contract, then such time period governs the duration of performance. See Childs v. Columbia, supra. The circumstances surrounding the agreement and the terms of the agreement itself indicate that the contract was to remain enforceable for an indefinite period:
"[N]o Decree obtained by either party shall in any way affect this Agreement or any of the terms, covenants, or conditions hereunder, this Agreement being unconditional and both parties intending to be legally bound thereby, except to such extend as may be herein limited with particular respect to any amount of alimony or child support."
The parties' use of the term on a temporary basis does not make the agreement too uncertain for enforcement. It is clear from the circumstances that the amount of payment was to remain constant until the execution of a subsequent agreement. The mere fact that the amount of future support payments is to be agreed upon in the future does not make the agreement invalid for indefiniteness and uncertainty as to the present certain terms. See generally 1 Corbin on Contracts § 97 (1963 & Supp.1971). The cases relied on by defendant do not establish a contrary proposition. What they indicate is that the enforceability of the unagreed upon terms may be too uncertain to be enforced where specific performance is sought. See Craven v. Williams, 302 F.Supp. 885 (D.S.C.1969).
Defendant's contention, that the parties' provision that the agreement would be revocable "as necessity may dictate" makes the agreement too indefinite, is without merit. Parties to a contract may expressly reserve a power to cancel or terminate the agreement upon a specified contingency or designated event. Helsby v. St. Paul Hospital and Casualty Company, 195 F.Supp. 385 (D.Minn.1961), aff'd, 304 F.2d 758 (8th Cir. 1962); 17 C.J.S. Contracts § 100(6), p. 809.
In Helsby, supra, the District Court held an employment contract terminable only "with cause" sufficiently certain to be enforceable. A power to revoke "as necessity may dictate" is also sufficiently certain to constitute a valid and enforceable term.
Defendant contends that the contract was one terminable at will and that no notice was required. This contention is also without merit.
The agreement by its express terms provided that it was to be revocable only "as necessity may dictate." It was not one terminable at will. Defendant's power to terminate the agreement was contingent upon the occurrence of a specified contingency. Until the contingency occurred, defendant had no power to terminate it. A fortiori, he could not terminate the agreement until he had shown the occurrence of the specified contingency. 6 Corbin on Contracts § 1266 (1962 & Supp.1964). Where an agreement extends for an indefinite duration and is only revocable upon the occurrence of a specified contingency, it is incumbent upon the party asserting the occurrence of the condition to notify the other party to the agreement of the occurrence. See generally 6 Corbin on Contracts § 1266 (1962 & Supp.1964). This defendant did not do. His attempted revocation of the agreement *48 was ineffective, and the trial court correctly found him liable for failing to pay in accordance with the contract's terms.
We find no error in the trial court's rulings as to defendant.

Plaintiff's Appeal
Plaintiff contends that the trial court erred in finding that the $700.00 monthly payments were for support and thus terminated upon her subsequent remarriage.
Section 9 of the agreement provides for the payment of $700.00 monthly. It does not specify whether the payments are for support or property settlement. It provides that the payment of money shall be "tax free"; but also provides that the parties "leave open the matter of any alimony or child support should a divorce action ever be instituted by one against the other." Parol evidence was admissible to ascertain the true meanings and intentions of the parties. Herndon v. Wardlaw, 100 S.C. 1, 84 S.E. 112 (1915). We find no error in the court's admittance of the parol evidence or in the court's determination that the payments were to cease upon remarriage.
Unless a contrary intention is expressed, a wife's remarriage terminates the husband's obligation to support her under a separation agreement which is silent on the question of the wife's remarriage. Annot., 48 A.L.R.2d 318 (1956). Here the agreement is noticeably silent on the question of the effect that the wife's remarriage would have. The trial court could properly find from the evidence presented that the payments were to terminate upon the subsequent remarriage of the spouse.
Plaintiff's final assignment of error is that the court erred in abating the $700.00 monthly payments by $250.00. We disagree.
Section 9 of the agreement expressly states that the plaintiff is to receive such funds tax free, and that all income taxes, if any, are to be paid by the husband. The agreement further provides that it is to be binding on the parties except as limited therein to alimony or child support. Language creating such an ambiguity may be clarified by looking at the subject matter and surrounding circumstances at the time the agreement was executed. Herndon v. Wardlaw, 100 S.C. 1, 84 S.E. 112 (1915); 3 Corbin on Contracts § 543 (1960 & Supp. 1971).
From the evidence presented at trial, the trial court could properly conclude that the parties intended that the monthly payments abate by $250.00 when the daughter no longer resided with plaintiff. We find no error.
The judgment entered below is affirmed as to both the plaintiff and defendant.
PARKER and HEDRICK, JJ., concur.
