(dissenting).
I dissent.
What started as a simple administrative request for a modification in child support has become a $32,000 windfall for Susan Houser. When Susan first contacted the Illinois Office of Child Support Services, she merely sought an increase in her current child support income of $135 per month. Susan only pursued the claimed arrearages after that office informed her that modifications and past-due collections were one process. Under the majority view, as a result of this bureaucratic intervention, she will now receive child support for periods when her *887children were: (a) not living with her and/or (b) adults. How can an equitable action for child support spawn such inequitable results? I am especially mystified by the majority opinion, because neither the divorce decree nor our child support statutes mandate such unfairness.
1. Whether the 1984 Divorce Decree provides for a lump sum or a pro rata distribution for child support?
According to the majority, the 1984 divorce decree is unambiguous and clearly provides for a monthly support award of $400 until all the children reach majority. The majority argues that our decision in Radigan v. Radigan, 465 N.W.2d 483 (S.D.1991), mandates this result. I disagree. Contrary to the majority’s statement, the language of the divorce decree in Radigan is not similar to the language in this case. In Radigan, the decree ordered the father to pay $750 per month in child support “until both children reach the age of majority or until further Order of the Court.” Id. at 485 (emphasis added).* Since the decree specifically required that “both children” reach majority, it is not surprising that this Court required father to pay $750 per month until his youngest child turned eighteen. In contrast, the divorce decree here does not include any explicit statement that Bob pay the $400 per month until all children reach majority. There lies the ambiguity. The decree simply provides:
... [Bob] shall pay as child support for and on behalf of the minor children of the parties the sum of $400.00 Dollars per month ... until said children shall attain the age of majority or until further order of the court.
It is unclear whether the support award is intended to remain at $400 until all children reach majority or whether support was intended to decrease by a one-third share as each child turned eighteen.
Because the agreement included in the divorce decree is ambiguous, the parties’ actions following the divorce are significant. We have written:
Another test to be applied in determining the meaning of a contract is the construction actually placed on the contract by the parties as evidenced by their subsequent behavior.
If the intention of the parties is not clear from the writing, then it is necessary and proper for the court to consider all the circumstances surrounding the execution of the writing and the subsequent acts of the parties. The construction given by the parties themselves to the contract as shown by their acts, if reasonable, will be accorded great weight and usually will be adopted by the court.
Malcolm v. Malcolm, 365 N.W.2d 863, 865 (S.D.1985) (citations omitted).
In this case, the actions of Bob and Susan after the divorce show that they intended to reduce support payments as the older children reached majority. Bob paid no support for his two older children after they turned eighteen years old on November 13, 1987, and June 6, 1992, respectively. Bob had previously reduced his support payments by roughly two-thirds, from $400 per month to $135 per month for his remaining minor child who resided with Susan. Susan accepted reduced payments for years without seeking an enforcement order. Therefore, the decree should be interpreted to coincide with the parties’ demonstrated intention to reduce child support payments as their older children reached age eighteen.
Pursuant to SDCL 25-5-18.1, child support obligations persist until the child attains eighteen years of age or until the child is nineteen years old if the child is a full-time student in a secondary school. Applying this statute to the parties’ agreement, Bob’s support obligation for his oldest child terminated in June 1988, when she had completed secondary school and had turned nineteen. Bob’s child support obligation for his middle child concluded at the end of June 1992, when she had graduated from high school and had turned eighteen.
2. Whether abatement under SDCL 25-7-6.14 was properly granted and calculated?
The majority accepts Susan’s claim that Bob should not be allowed an abatement of *888child support, because he did not promptly enforce his right to receive it in a court of law. According to the majority, Bob should have made a $400 payment during the first month the children resided with him, and then immediately applied to the court for an abatement of subsequent child support obligations. Ironically, the majority does not impose the same prompt enforcement requirements on Susan. Instead, the majority endorses Susan’s claim to additional child support when she complacently accepted a lower amount for nine years. In an equitable action, one party should not be permitted to sit on her rights while another party is penalized for doing so. If we allow Susan to wait nine years before pursuing child support arrearages, as we must, we should certainly permit Bob to respond with an abatement claim covering the same period.
The majority further argues that abatement should only be permitted to reduce future obligations. According to the majority, applying abatement to past obligations amounts to retroactive modification. The majority relies on a footnote in the 1988 Report of the South Dakota Commission on Child Support:
For example, if a parent exercises visitation in June for the entire month, he or she would be entitled to the abatement in the July child support. If a 50 percent abatement were allowed, the July payment would be reduced by 50 percent.
Whalen v. Whalen, 490 N.W.2d 276, 282 (S.D.1992) (citing Report of the South Dakota Commission on Child Support at 15 n. 9 (1988)). However, the Commission’s example simply demonstrates that a parent is not entitled to abatement until the child has spent more than 29 consecutive days with the parent. Rather than imposing a requirement that all abatement come from future payments, the Commission simply illustrated the importance of after-the-fact proof of visitation. Furthermore, abatement is appropriate where the obligation has simply accrued; the obligation need not have been paid in full. See Whalen, 490 N.W.2d at 282 (“Use of the abatement language [in SDCL 25-7-6.14] indicates the legislature contemplated the obligation would have already accrued when the obligor spouse seeks abatement.”). Although forward-reaching abatement is certainly an option for a non-custodial parent, see Sjolund v. Carlson, 511 N.W.2d 818 (S.D.1994), abatement of past due payments is still proper.
Neither Susan nor the majority dispute the extended periods of time during which one or more of the children resided with Bob. Giving SDCL 25-7-6.14 prospective application, the trial court properly permitted abatement of child support during these periods.
I would affirm the trial court on Issues 1 and 2.
I am authorized to state that AMUNDSON, J., joins in this dissent.
che majority quotes our conclusion in Radigan rather than the language of the decree.