MAASSEN, Justice.

Allison Mendel and John J. Sherman, Mendel Colbert & Associates, Inc., Anchorage, for Appellant and Cross-Appellee.

Rhonda F. Butterfield, Wyatt & Butterfield, LLC, Anchorage, for Appellee and Cross-Appellant.

Before: Stowers, Chief Justice, Winfree, Maassen, Bolger, and Carney, Justices.

OPINION

MAASSEN, Justice.

I. INTRODUCTION

The parents of a young son divorced in 2011, agreeing that the mother would have primary physical custody during the school year. The father moved to California. When the child began experiencing behavioral problems, the parents agreed to switch custody for a few years, giving the father primary physical custody in California during the school year. The parties signed a custody modification agreement to this effect in December 2014, including both parents’ waivers of child support, but they did not file the agreement in court.

The next year, following a dispute over the mother’s visitation, both parties sought a judicial resolution of custody. The father went to a California court seeking to make the 2014 change in custody permanent. The mother went to Alaska superior court seeking to enforce the original 2011 agreement that gave her primary physical custody. The Alaska court asserted jurisdiction; it ultimately modified physical custody in favor of the father but maintained the parents’ joint legal custody. The court also modified child support, ordering the mother to pay child support effective from the date the father first sought to modify custody in California.

The mother appeals the physical custody and child support orders, challenging among other things the child support order’s effective date. The father cross-appeals, challenging the decision on joint legal custody. We affirm the court’s custody and child support orders, concluding that they are well supported by the evidence and that the court did not abuse its discretion in selecting the child support order’s effective date.

II. FACTS AND PROCEEDINGS

A. Facts

Darcey and Matthew Geldermann married in 2005 and had a son in 2008. They divorced in February 2011. The superior court approved a custody agreement that established joint legal custody but gave primary physical custody to Darcey, with Matthew receiving visitation during summers and holidays. Matthew thereafter moved to Washington, spent time as a contractor in Iraq, and eventually settled in California.

1. Custody experiment

The parties agree that their son exhibited behavioral problems when he started kindergarten in 2013; psychologists diagnosed him with autism, atypical attention deficit and hyperactivity disorder, unspecified anxiety disorder, and social communication disorder. To deal with these problems Darcey and Matthew agreed to try a temporary custody switch: their son would live primarily with Matthew in California for two and a half years and visit Darcey over summers and holidays. Around December 2014, when the child was six, the parties negotiated a new custody agreement that acknowledged the switch, but they did not formalize their new agreement in court.

The informal new arrangement worked well for about 10 months. The child started school in California in January 2015. Matthew hired a nanny and established a rigorous routine, including many extracurricular activities. The child had fewer behavioral problems, and his grades improved. Matthew claims that his son "thrived" under the new arrangement.

But the arrangement broke down in the fall of 2015 after a dispute over a planned visit by Darcey. The superior court found that the dispute generated "some angry emails," including threats by Darcey that she would retrieve the child from California and make it difficult for Matthew to see him again. The parties filed custody-related motions in both California and Alaska courts; meanwhile the child remained with Matthew in California.

While both cases were pending, the parties struggled to maintain civil communication with each other. The California court awarded Darcey a visit with her son in November 2015 when she was there for a hearing; the visit was not a success, though the parties disagree on why. In March 2016 Matthew moved the child to a new school following a bullying incident and failed to immediately inform Darcey of the change. In April the Alaska superior court, retaining jurisdiction, awarded Darcey spring-break visitation, requiring only that she provide an itinerary. The parties disagreed about how specific the itinerary needed to be, and Darcey ultimately canceled her visit. The superior court described the "failure" of the spring break visit as "disturbing" and "highly unfortunate."

2. Child support

As part of their 2011 divorce, the parties’ court-approved settlement agreement provided that Matthew would pay Darcey child support. But when their son moved in with Matthew in December 2014, "[t]he parties agree[d] that neither party [would] owe child support to the other party as a result." In January 2015, accordingly, Darcey withdrew from the collection services provided by Alaska’s Child Support Services Division (CSSD). But when their current dispute arose, Darcey again requested CSSD assistance, and Matthew received a letter from CSSD in October 2015 directing that he pay Darcey the child support required by the 2011 agreement.

B. Proceedings

1. Custody

In early October 2015 Matthew filed an ex parte custody action in California, seeking a transfer of jurisdiction from Alaska, a temporary emergency custody order, and a modification of custody to reflect the parties’ 2014 agreement giving him primary physical custody. Shortly thereafter Darcey commenced the current Alaska proceedings; she filed a motion to enforce the original 2011 custody arrangement that gave her primary physical custody. The Alaska superior court conferred with the California court, held an evidentiary hearing on jurisdiction in January 2016, concluded that neither forum was convenient for both parties, and ultimately decided to retain jurisdiction in Alaska.

Matthew had not filed a motion in Alaska to modify custody. The superior court concluded, however, that his opposition to Darcey’s motion to enforce the 2011 order amounted to, "at the very least, an implied custody modification request," and it scheduled a trial on both physical and legal custody. The parties presented a number of witnesses, including themselves, their respective new spouses, their son’s California nanny, and Darcey’s mother.

After hearing the evidence, the court denied Darcey’s motion to enforce the 2011 order and awarded Matthew primary physical custody during the school year. Darcey was given summer vacations and alternating holidays as well as "generous and liberal, unrestricted, unsupervised visits with [the child], whenever she is in the same geographic location." Although the court found significant communication problems between Darcey and Matthew, it continued joint legal custody. Darcey sought reconsideration, claiming that custody modification was inappropriate absent a formal motion from Matthew. The court denied the motion, explaining that custody and visitation had been "always clearly at issue."

2. Child support

In October 2015 Darcey filed a motion to reduce to judgment past-due amounts of child support calculated under the 2011 order. In early December Matthew sought to stay enforcement of those child support provisions, and in January 2016 the court issued an order precluding collection of child support arrears from December 2014 until it ordered otherwise.

Matthew did not file any other documents related to child support until May 18, 2016, following the court’s custody decision, when he filed a proposed child support order. He proposed an effective date of "November 1, 2015, which is the first of the month after [Darcey] was formally served with notice of [Matthew’s] custody action in California." Darcey objected, arguing that Alaska Civil Rule 90.3(h)(2) prohibited a retroactive date because Matthew had never filed a motion to modify child support.

The court signed a new child support order on June 1, 2016, requiring Darcey to pay support to Matthew. The order used Matthew’s proposed effective date of November 1, 2015. Darcey sought reconsideration, which was denied.

Both parties appealed. Darcey seeks review of the physical custody and child support orders, and Matthew challenges the award of joint legal custody.

III. STANDARDS OF REVIEW

"Superior courts have broad discretion in child custody decisions, and we will reverse only if findings of fact are clearly erroneous or if the superior court abused its discretion." "A factual finding is clearly erroneous when a review of the record leaves [us] with a definite and firm conviction that the superior court has made a mistake." "An abuse of discretion exists where the superior court ‘considered improper factors in making its custody determination, failed to consider statutorily mandated factors, or assigned disproportionate weight to particular factors while ignoring others.’ " "Additionally, an abuse of discretion exists if the superior court’s decision denied a substantial right to or substantially prejudiced a party." "We review an award of child support for abuse of discretion." But "[w]hether the superior court applied the correct legal standard to its child support determination is a question of law that we review de novo." Finally, "[t]he adequacy of the notice and hearing afforded a litigant in child custody proceedings involves due process considerations," which we also review de novo.

IV. DISCUSSION

A. The Superior Court Did Not Abuse Its Discretion By Finding A Substantial Change In Circumstances For Purposes Of Modifying Physical Custody.

" Alaska Statute 25.20.110 authorizes courts to modify child-custody and visitation awards if (1) there has been a change in circumstances that justifies modification and (2) the modification is in the best interests of the child." "Once the movant meets [the initial] burden [of changed circumstances], he or she is entitled ‘to a hearing to consider whether, in light of such changed circumstances, it is in the child’s best interests to alter the existing custodial arrangement.’ "

Darcey challenges only the court’s decision on the first element of the modification test: that there was a change in circumstances substantial enough to justify a modification of custody. We have held that "[a] change in circumstances is unlikely to be substantial enough to ‘overcome our deep reluctance to shuttle children back and forth between parents’ unless the change affects the children’s welfare and ‘reflect[s] more than mere passage of time.’ " Whether changed circumstances justify modification "is heavily fact-intensive" and "appropriately gauged by its effect on the child." "The change in circumstances ‘must be demonstrated relative to the facts and circumstances that existed at the time of the prior custody order that the party seeks to modify.’ "

The superior court in this case had to decide whether circumstances had substantially changed since the first custody agreement in 2011. In deciding that they had, the court relied on the change that prompted Darcey and Matthew to shift custody in 2014: "[T]here ha[s] been a change in circumstances ... that was recognized by the parties when they agreed that [the child] would reside, at least temporarily, with his father." It is undisputed that the child had behavioral problems in Alaska, and both parties agreed it would benefit him to live with Matthew in California for several years. Matthew had also had a change in employment that allowed for a more permanent residence, and both parents had remarried. The superior court did not abuse its discretion in concluding that these new circumstances, in combination, amounted to a substantial change. Citing McLane v. Paul , Darcey argues that, as a matter of law, her temporary agreement with Matthew should not qualify as a substantial change in circumstances because "parties should be allowed to experiment with new custodial arrangements without the fear that they will automatically become permanent." But the superior court did not point to the "new custodial arrangement" as the substantial change; rather, the court relied on the underlying circumstances that prompted the new custodial arrangement. The fact that the parties themselves recognize the substantiality of a change in circumstances and agree tentatively on how to deal with it cannot foreclose the court from recognizing the same thing.

B. The Superior Court Did Not Violate Darcey’s Due Process Rights By Considering A Modification of Custody Without A Formal Motion From Either Party.

Darcey argues that Matthew’s failure to file a motion to modify custody in Alaska prevented her from understanding "what the alleged change in circumstances was," thus violating her due process rights to notice and a hearing. "Procedural due process under the Alaska Constitution requires ‘notice and opportunity for hearing appropriate to the nature of the case.’ " "To comply with due process, notice must be given sufficiently in advance of scheduled court proceedings so that the parties have a reasonable opportunity to prepare." We have explained that "[i]t is essential to contested custody proceedings that the parties be afforded a hearing which grants them the opportunity to present the quantum of evidence needed to make an informed and principled determination." But we have held that even without a formal motion from either party, "the trial court may decide [custody] issues on its own motion, as long as a party has raised them and both sides have the opportunity to present full testimony."

Darcey argues that in the absence of a formal motion to modify custody, she "lacked notice as to the disputed issues and therefore had no ability to prepare to respond to Matthew’s case." Darcey acknowledges that she received "late notice" at the trial call that modification was at issue, but she argues that because she was unrepresented, the absence of a formal motion "deprived her of any understanding of the issues to be addressed at trial, or the case Matthew intended to present against her retaining custody."

We have found due process violations in several cases when a parent lacked notice that permanent custody was at issue or the hearing was insufficient to determine the child’s best interests. In Siekawitch v. Siekawitch , on the other hand, we rejected a father’s due process claim because of his actual "notice that [the mother] sought equal time with the children." Although the mother had not filed a motion to modify physical custody, she had sought equal time through a change in visitation "in her memorandum in support of her motion for a specific custody schedule, in her reply to [the father’s] opposition to the motion, and in her proposed visitation order." Because the father’s pretrial motions and his conduct at trial indicated that he "was aware of [the mother’s] intentions" to seek equal time, we concluded that the father had sufficient notice of a possible modification to satisfy due process.

Darcey’s case is not meaningfully distinguishable from Siekawitch . First, Matthew’s filings and the court’s comments clearly indicated that custody modification was at issue. Although Matthew did not file a motion to modify custody in Alaska, he did file one in California, served on Darcey on October 8, 2015. And Matthew’s motion to transfer jurisdiction, filed two weeks later in Alaska, clearly conveyed his intent to modify custody in whichever state took jurisdiction. His January 14, 2016 reply on the jurisdiction issue also discussed which state should "hear the custody modification action."

The superior court’s interactions with Darcey clearly indicated its intent to consider modifying custody. When the court decided to retain jurisdiction in January 2015, it stated that modification would be the subject of a future evidentiary hearing and asked the parties when they would be ready for it. At trial call, 15 days before the evidentiary hearing, the court clearly stated that physical and legal custody were at issue. And at the beginning of the hearing the court again stated that the "hearing [was] on custody modification."

Finally, the record supports a conclusion that Darcey understood from the outset that Matthew was seeking to modify custody. She acknowledged that she knew Matthew was attempting to modify custody in California when she sought the protection of Alaska’s courts: "Matthew ... is trying to modify custody in California. He filed our Alaska custody order in the court of Orange County and is trying to modify it through the State of California and not Alaska." When the court advised her at the trial call to "be prepared for the court to rule on who should have physical and legal custody, okay?," Darcey responded "Okay." Her trial brief, filed the day before trial, addressed "Custody Modification/Enforcement," argued that "[t]here should be no modification of custody granted, and only enforcement of the existing order," and addressed the statutory best interests factors. We conclude that the superior court did not err in concluding that Darcey had sufficient notice of a possible custody modification. Darcey also had a sufficient opportunity to be heard; the hearing enabled the superior court to make a principled custody decision. Both parties presented evidence on the best interests factors. Darcey’s witnesses addressed her ability to care for the child and contrasted it with Matthew’s. She cross-examined Matthew about their relative abilities to foster their son’s relationship with the other parent, Matthew’s home environment, and his relationship with his family. She does not point to any additional evidence she would have presented if she had received earlier or more specific notice. Even taking Darcey’s self-represented status into account, we conclude that her due process rights were not violated.

C. It Was Not Error To Select November 1, 2015, As The Effective Date Of The Modified Child Support Award.

We have observed that "a significant modification of the physical custody schedule is likely to require a new child support determination, regardless of whether a parent requests it." But whether a parent requested the modification is relevant to whether the new order can have anything but prospective effect. Although Alaska Civil Rule 90.3(h)(2) prohibits most retroactive modifications of "[c]hild support arrearage," the rule further provides that a modification may be made effective "on or after the date that a motion for modification ... is served on the opposing party." Darcey points out that Matthew never filed a motion to modify child support, and she argues that the superior court violated the retroactivity bar when it modified child support effective November 1, 2015, the first day of the month after Matthew filed his motion for modification of custody in California.

In light of the purposes of Rule 90.3(h)(2) and the realities of custody litigation, we reject Darcey’s argument. We conclude that when, as here, a parent moves to modify custody based on an existing, de facto custody change, the prohibition on retroactivity does not bar the superior court from making a corresponding change in child support effective from the date the modification motion is served on the opposing party.

A baseline principle is that child support calculations "must be based on the custody actually ordered, not exercised." We explained in Turinsky v. Long that "[i]f the parties do not follow the custody order, they should ask the court to enforce the custody order or should move to modify the child support order." Although modified child support may be calculated from the date a motion to modify child support is served on the opposing party, "[w]e have read Rule 90.3(h)(2) to prohibit increases or decreases in child support effective before" that date. Superior courts have discretion to choose a later date, but "the motion service date" is the earliest available date and "should be the preferred effective date." "[T]he superior court should exercise its discretion in selecting a different effective date only if it finds good cause for doing so."

We recognize that our case law has not been entirely consistent on the issue of what "motion for modification" can be used to satisfy Rule 90.3(h)(2). We have not allowed the "functional equivalent" of a modification motion to determine the effective date of a support order. In Millette v. Millette we held that the earliest date the court could use for a modification of child support was the date the father received notice of the mother’s motion for modification of child support, which was months after the superior court had ordered a temporary change in custody; we noted that neither the court’s custody order nor the mother’s motion seeking modification of custody mentioned child support. We explained that a formal motion to modify child support was required "even where the circumstances have materially changed and a modification would likely be permitted upon motion." "In such cases, the burden remains on the non-custodial parent to file a motion for modification of child support." We have further observed that "allowing the court to alter [a parent’s] obligation absent the filing of a motion for modification would undermine Rule 90.3 ’s stated goal of ensuring predictability in determining the amount of child support to be paid."

On the other hand, we recently indicated that a modification of child support could be effective as of the date of the mother’s motion to modify custody where, as in Millette , the motion was silent on child support. But because the father in that case "concede[d] that [the mother’s] motion to modify custody satisfied the [retroactivity] provision of Alaska R. Civ. P. 90.3(h)(2)," it was unnecessary for us to decide whether Rule 90.3(h)(2) was actually satisfied.

Harmonizing these cases requires us to again look at the purposes behind the retroactivity bar. Rule 90.3(h)(2) restates the federal prohibition of retroactivity found in the Bradley Amendment, which "specifies what laws a state must have in order to receive federal funds" for child support enforcement activities. Among the Bradley Amendment’s requirements is that states have procedures ensuring that child support payments not be "subject to retroactive modification ... except that such procedures may permit modification with respect to any period during which there is pending a petition for modification, but only from the date that notice of such petition has been given ... to the obligee or (where the obligee is the petitioner) to the obligor." Discussing the policy reasons behind the federal retroactivity bar in Karpuleon v. Karpuleon , we noted commentators’ concerns that evidence was sometimes not "easily attained or available" to support retrospective arguments about changed circumstances and that obligor parents should be required to act diligently when petitioning for reductions in their child support payments.

The United States Department of Health and Human Services (DHHS), responding to comments on the Bradley Amendment’s implementation, repeatedly emphasized that it is the obligation of the non-custodial parent—who "is in the best position to know of a change in circumstances"—"to take action promptly to seek modification of a support obligation." Changed circumstances that trigger the obligor’s responsibility to act "might include" the fact that "the child [has gone] to live with the obligor." As relevant here, DHHS reiterated: "We do not ... believe that ... even a change of actual custody of the child should result in a modification of support liability unless the court or administrative authority is duly notified and sanctions such modification." And the required notice is the formal notice required by state jurisdictional rules:

We believe Congress intended that "date of notice" or "date notice is given" should be construed literally and in terms of acquiring personal jurisdiction over the other party. State law provides rules to determine when personal jurisdiction is acquired by service of notice of an action. Under these provisions, the "date of notice" or "date notice is given" should be interpreted by the State in the same way as it is generally applied to commence other civil litigation within the State. State law regarding the establishment of the date of notice that a petition has been filed dictates when the modification may be effective.^{[ ]}

The federal agency comments do not interpret the Bradley Amendment as dictating the title, form, or substance of the "motion for modification" necessary to satisfy the retroactivity bar. They focus primarily on the importance of two factors: (1) notice that child support is at issue, and (2) that the notice be formal—that is, that it involve the court. In our view, a motion for modification of custody based on an existing, de facto change in physical custody will usually provide the other parent the same "fair warning that support may change" as the parent would receive from an explicit motion to modify child support. The requirements of federal law—focusing on notice and formal process—remain satisfied if the superior court has the flexibility to choose the date of that custody motion as the effective date for a modification of child support. In this case, Matthew filed an ex parte custody action in California in early October 2015 that sought, among other things, a modification of custody to reflect the parties’ 2014 agreement giving him primary physical custody. Darcey acknowledges that Matthew informed her in late September that he was taking this action, and he later filed in Alaska a California "Proof of Personal Service" that showed she had been served with the pleadings on October 8, 2015. At that time the child was already living with Matthew, and Darcey acknowledges she had actual notice of Matthew’s intent to formalize their de facto custody arrangement by seeking to modify custody in California.

We conclude that the superior court did not abuse its discretion when it selected November 1, 2015—the first date of the month immediately following the filing of the California custody action—as the effective date of the new child support order.

D. The Superior Court Did Not Abuse Its Discretion By Continuing Joint Legal Custody.

Matthew argues in his cross-appeal that the superior court erred by refusing to modify joint legal custody despite its finding—and both parties’ testimony—that their communication had broken down. We conclude that this was not an abuse of discretion.

"Joint legal custody is preferred" but "is only appropriate when the parents can cooperate and communicate in the child’s best interest." Thus, "the test for revaluating the propriety of joint legal custody is whether or not the parties can cooperate and communicate regarding the children."

Here, the superior court acknowledged "concerns about the parties’ ability to communicate" but also found that Darcey and Matthew had communicated "somewhat effectively" before the current issues arose in the fall of 2015. The court discussed its "hope that the parties will be able to communicate again, once the current issues are resolved." We conclude that it was not manifestly unreasonable under the circumstances for the court to base its decision on that hope, grounded in turn in its direct experience with the parties.

V. CONCLUSION

We AFFIRM the superior court’s custody and child support orders.