¶ 1. Bradley M. Palmer-sheim appeals a circuit court order denying his motion for modification of custody and physical placement of Palmersheim's minor son. The circuit court found Palmersheim failed to establish a significant change in circumstances from the previous custody and physical *318placement order and also found Palmersheim failed to rebut the presumption that the current custody and physical placement arrangement was in the child's best interest under Wis. Stat. § 767.325(l)(b) (2001-02).1 Because the record does not support the circuit court's finding of no significant change in circumstances, we conclude the circuit court erred. However, because the circuit court proceeded to the second step in the analysis, that is, whether Palmersheim established that his proposed changes in custody and placement were in the child's best interest, we also conclude the circuit court's error was harmless. We further conclude that the circuit court applied the correct legal standard in determining the best interest of the child and that Palmersheim's equal protection rights were not violated. We therefore affirm the circuit court's order.
BACKGROUND
¶ 2. Palmersheim and Leanne M. Abbas are the parents of Nicholas, born October 6, 1991. A stipulated paternity judgment was entered on February 17, 1992, awarding sole legal custody to Abbas and periods of physical placement with Palmersheim "as agreed between the parties." At the time of this paternity judgment, Wis. Stat. § 767.51(3) (1991-1992) provided, in relevant part, "Unless the court orders otherwise,. . . the mother shall have sole legal custody of the child."
¶ 3. In August 2001, Palmersheim filed a motion to modify the original paternity judgment, seeking joint custody and shared physical placement. A hearing was held on this motion on June 10, 2002. Both Palmer-sheim and Abbas testified, as did Teri Phalin, a certified *319social worker who had completed a home study at the request of the guardian ad litem. Phalin testified, and her report states, the child is well adjusted, is doing well in school and is participating in both sports and an accelerated learning program. Phalin recommended joint custody, as did the guardian ad litem.
¶ 4. The circuit court, applying Wis. Stat. § 767.325(l)(b), denied Palmersheim's motion, finding that (1) Palmersheim failed to establish a substantial change in circumstances substantially affecting legal custody and physical placement since the entry of the previous order, and (2) Palmersheim failed to rebut the presumption that the current allocation of decision making under the legal custody order and the continuation of the child's primary physical placement with Abbas are in the best interest of the child.
¶ 5. On December 17, 2002, Palmersheim moved for reconsideration of the circuit court's December 4, 2002 order, claiming his constitutional right to equal protection was violated by application of the Wis. Stat. § 767.325(l)(b)2 presumptions. The circuit court denied his motion stating it is not unfair to presume to continue the status quo after eleven years have passed since the original custody and placement order and Palmersheim should not be permitted to interject a constitutional argument not presented at the motion hearing six months earlier. Palmersheim appeals.
DISCUSSION
Substantial Change in Circumstances
¶ 6. Whether to modify a custody or physical placement order is directed to the circuit court's sound discretion. Keller v. Keller, 2002 WI App 161, ¶ 6, 256 *320Wis. 2d 401, 647 N.W.2d 426. We affirm a circuit court's discretionary determination when the circuit court applies the correct legal standard to the facts of record and reaches a reasonable result. Id. Our task as a reviewing court is to search the record for reasons to sustain the circuit court's exercise of discretion. Id. However, when a party argues that the circuit court erroneously exercised its discretion by applying an incorrect legal standard, we independently review that issue of law. Id.
¶ 7. Wisconsin Stat. § 767.325(l)(b)2 provides that, after two years, a circuit court may substantially modify custody or physical placement if the modification is in the child's best interest and there has been a substantial change in circumstances since the entry of the last custody and placement order. Section § 767.325(l)(b)2 establishes a rebuttable presumption that
a. Continuing the current allocation of decision making under a legal custody order is in the hest interest of the child.
b. Continuing the child's physical placement with the parent with whom the child resides for the greater period of time is in the best interest of the child.
¶ 8. Whether there is a substantial change in circumstances is a mixed question of law and fact. See Harris v. Harris, 141 Wis. 2d 569, 574, 415 N.W.2d 586 (Ct. App. 1987). The circuit court's findings of fact regarding an alleged change of circumstance since the last custody and placement order will not be disturbed unless clearly erroneous. Rosplock v. Rosplock, 217 Wis. *3212d 22, 33, 577 N.W.2d 32 (Ct. App. 1998). However, whether a substantial change in circumstances has occurred is a question of law. Keller, 256 Wis. 2d 401, ¶ 7. Because the circuit court's legal determination is mixed with its factual findings, we give weight to the circuit court's decision. Rosplock, 217 Wis. 2d at 33 (citation omitted).
¶ 9. Palmersheim argues that the circuit court erred by determining there was not a substantial change in circumstances since the entry of the last order affecting legal custody or physical placement. A substantial change of circumstances requires that the facts on which the prior order was based differ from the present facts and the difference is sufficient to justify the circuit court's consideration of modification. Keller, 256 Wis. 2d 401, ¶ 7. We are not bound by a circuit court's determination of whether there' was a significant change in circumstances. Id.
¶ 10. We conclude the evidence of record does not support the circuit court's finding of no substantial change in circumstances since the entry of the last order affecting legal custody or physical placement. Apparently the circuit court focused on events that occurred in the years just prior to the hearing in this matter, which, standing alone, do not show a substantial change in circumstances. However, when the entire relevant time period is considered, the facts clearly show a significant change in circumstances.
¶ 11. At the time of the last custody and placement order (the 1992 paternity judgment and order), Nicholas was four months old. Palmersheim and Abbas resided in the home of Palmersheim's parents in Wisconsin Rapids, Wisconsin. Palmersheim was a student *322at the University of Wisconsin-Stevens Point and Abbas was unemployed, receiving government assistance in the form of financial aid, food stamps and Medical Assistance. Palmersheim was able to be with his son every day and be a full-time parent.
¶ 12. Approximately six to eight months after the custody and placement order was issued, Abbas and Palmersheim broke up and Abbas moved to an apartment in Plover, Wisconsin, with Nicholas. Because of the custody and placement order, Abbas had sole legal custody and primary physical placement with periods of physical placement to Palmersheim as agreed between the parties. Palmersheim had irregular contact with Nicholas during the next several years. Beginning around 1994, Palmersheim had physical placement of Nicholas approximately every other weekend with perhaps one additional night during the week. Palmer-sheim also had extended periods of visitation during the summer months.
¶ 13. Today, Palmersheim and Abbas reside in different communities approximately forty-five minutes apart, have successful careers and are in commit,ted relationships with other persons. Nicholas is now twelve years old, well adjusted and successful in school. Palmersheim, at the time of the motion hearing in this case, had physical placement of Nicholas every other weekend from Friday until Monday mornings and overnight every Thursday. The parties alternated placement on holidays.
¶ 14. The facts upon which the prior order was based are substantially different from the present facts and this difference is sufficient to justify modification of the previous order. This is particularly true with respect to the amount of time Palmersheim has with Nicholas. At the time of the original custody and placement order, Nicholas lived with both parents. *323Palmersheim saw Nicholas every day. At the time of the hearing in this case Palmersheim saw Nicholas only every other weekend and every Thursday night. This change is substantial and material. We conclude the circuit court erred by finding Palmersheim had not established a significant change in circumstances.
¶ 15. However, we also conclude the circuit court's error is harmless. Despite its conclusion of no significant change in circumstances, the circuit court proceeded to analyze the evidence under Wis. Stat. § 767.325(l)(b)2 to determine whether Palmersheim's proposed changes in custody and placement were in Nicholas's best interest. On appeal, Palmersheim asserts the circuit court used an erroneous legal standard when determining the best interest of the child. Palm-ersheim raises no objections to the circuit court's factual findings.2 We conclude the circuit court applied the correct legal standard and we affirm.
Best Interest of the Child
¶ 16. We now determine whether the circuit court applied the correct legal standard in determining *324whether the proposed changes in custody and placement were in Nicholas's best interest. Palmersheim argues the circuit court erred by applying the status quo presumption as provided by Wis. Stat. § 767.325(l)(b)2 rather than the joint legal custody presumption as required by Wis. Stat. § 767.24(2)(am).3 Palmersheim asserts Wis. Stat. § 767.325(5m),4 which requires the circuit court to consider whether the proposed modification of custody and placement is in the child's best interest "in a manner consistent" with § 767.24, obligates the court to presume that joint legal custody is in the child's best interest. Section 767.24 is the statute governing initial determinations of legal custody and physical placement. In essence, Palmersheim argues, the status quo presumption does not apply in modification proceedings. He claims that the joint legal custody presumption is the "new" rebuttable presumption applicable in all cases, including custody and placement modification proceedings.5 We disagree.
*325¶ 17. The question before us is one of statutory interpretation, which we decide de novo. See Truttschel v. Martin, 208 Wis. 2d 361, 364-65, 560 N.W.2d 315 (Ct. App. 1997). Our primary objective in statutory interpretation is to discern the intent of the legislature. See Anderson v. City of Milwaukee, 208 Wis. 2d 18, 25, 559 N.W.2d 563 (1997). To ascertain legislative intent, we first look to the statute's plain language. Id. A statute is ambiguous when it is capable of being understood in two or more different senses by reasonably well-informed persons. State v. Setagord, 211 Wis. 2d 397, 406, 565 N.W.2d 506 (1997).
¶ 18. This case requires us to construe the statutory construction of two related statutes. The entire section of a statute and related sections are to be considered in its construction or interpretation; we do not read statutes out of context. Kerkvliet v. Kerkvliet, 166 Wis. 2d 930, 939, 480 N.W.2d 823 (Ct. App. 1992). In determining the meaning of any single phrase or word in a statute, it is necessary to look at it in light of the whole statute and related sections. Id. Statutes relating to the same subject matter are to be construed together and harmonized. Id. We are to interpret statutes so as to effectuate the purpose of the whole act; a construction which will defeat the manifest object of the act is disfavored. Id. at 940.
¶ 19. Standing alone, neither Wis. Stat. § 767.24(2)(am) nor Wis. Stat. § 767.325(l)(b) is ambiguous. However, these statutes are ambiguous in their interaction. There is an inherent conflict between *326these statutes when applied to custody and physical placement modification petitions or motions. Which presumption applies under a particular circumstance is tricky business, especially in the case where neither party is successful in rebutting either presumption.
¶ 20. Before Wis. Stat. § 767.325(5m) was created, Wis. Stat. § 767.24 applied only to initial determinations of legal custody and physical placement and § 767.325 applied only to custody and placement modification proceedings. See Kerkvliet, 166 Wis. 2d at 941, 943-44. No ambiguity existed. However, ambiguity now exists by the creation of § 767.325(5m), which requires courts to construe § 767.325 "in a manner consistent with s. 767.24." This language could be construed as requiring courts to incorporate all aspects of § 767.24 by reference into § 767.325 in determining custody and placement modification motions. Another reasonable interpretation of this phrase is that courts are required to consider those aspects of § 767.24 plainly applicable to modification proceedings, such as § 767.24(5), which provides the factors courts must consider in determining custody and placement at both initial and modification proceedings. Moreover, the language of Wis. Stat. § 767.24(2)(am) does not support applying this section to modification proceedings. To further complicate matters, the legislature did not explicitly repeal Wis. Stat. § 767.325(l)(b), thereby creating the possibility that two rebuttable, but diametrically opposed, presumptions are to be applied in custody and placement modification determinations.
¶ 21. Palmersheim argues "in a manner consistent with s. 767.24" means the presumption of joint legal custody as stated in Wis. Stat. § 767.24(2) (am) must be applied in all custody modification proceedings. Abbas argues that the legislature's decision not to repeal the status quo presumption in Wis. Stat. *327§ 767.325(l)(b)2 shows the legislature did not intend to require courts to presume that joint legal custody was in the child's best interest in modification proceedings. What the legislature intended by this change is not so clear.
¶ 22. There is no dispute that the legislature, by enacting Wis. Stat. § 767.24(2)(am), intended to create a rebuttable presumption in favor of joint legal custody applicable in all initial custody proceedings. What is not clear is whether the legislature intended this presumption to apply in custody modification determinations. Moreover, were we to conclude that the legislature intended to apply the presumption of joint legal custody to custody modification actions, it is clear that the legislature failed to achieve its objective.
¶ 23. Because the legislature failed to specifically repeal the status quo presumption, we must construe the statutes in a manner so as to resolve the ambiguities. We bear in mind that where statutes relate to the same subject matter, they should be read together and harmonized if possible. Kerkvliet, 166 Wis. 2d at 939. To harmonize the two statutes, we must read them together in a way that gives each full force and effect. City of Milwaukee v. Kilgore, 193 Wis. 2d 168, 184, 532 N.W.2d 690 (1995).
¶ 24. We conclude that the most reasonable interpretation of Wis. Stat. § 767.24(2)(am) is that the presumption that joint legal custody is in the child's best interest applies only in initial legal custody determinations, not in modification determinations.6 Were we to conclude that the § 767.24(2) (am) presumption *328favoring joint legal custody applied to a Wis. Stat. § 767.325(1)(b) motion for modification of custody and physical placement, such a conclusion would, in essence, eliminate the § 767.325(l)(b) presumption favoring the status quo. We will not interpret a statute in a way that results in the judicial nullification of a statute. See Phillips v. Wisconsin Pers. Comm'n, 167 Wis. 2d 205, 217, 482 N.W.2d 121 (Ct. App. 1992). Furthermore, this interpretation would abrogate the clear legislative preference for continuity in custody and placement. The legislature raised the bar for parties wishing to upset the status quo by enacting § 767.325(l)(b) because changes in custody and physical placement can be hard on children and change is not desirable unless supported by good reason. We conclude the legislature did not intend to eliminate the preference for continuity in custody and placement as codified in § 767.325(l)(b) by enacting § 767.24(2)(am) and Wis. Stat. § 767.325(5m).
¶ 25. In addition, the modification statute is inconsistent with a presumption favoring joint legal custody since it favors continuing the status quo regardless of whether the initial order awarded joint legal custody. Under Palmersheim's view, the status quo presumption would only apply when joint legal custody is the status quo. Moreover, even if the legislature intended to eliminate the presumption of maintaining the status quo in *329modification actions, the legislature failed to clearly signal its intentions by not specifically repealing Wis. Stat. § 767.325(l)(b).
¶ 26. Furthermore, it is presumed that the legislature acts with knowledge of existing statutes when enacting legislation. H.F. v. T.F., 168 Wis. 2d 62, 69 n.5, 483 N.W.2d 803 (1992). We must presume the legislature did not intend to create a conflict between these two statutes. Id. at 69-70 n.5. Thus, it is reasonable for us to assume that when the legislature enacted Wis. Stat. § 767.325(5m) requiring the circuit court, in all custody and placement modification actions, to make its determination "in a manner consistent" with Wis. Stat. § 767.24, the legislature was aware of the more specific status quo presumption required by Wis. Stat. § 767.325(l)(b).
¶ 27. Finally, because Wis. Stat. § 767.325(5m) is general and, as it relates to this topic, at best, ambiguous, and Wis. Stat. § 767.325(l)(b)2 is specific and clear with respect to the presumption to be applied in custody and placement modification proceedings, the specific language trumps the general language. See Brown County v. State Pub. Defender, 167 Wis. 2d 168, 174, 482 N.W.2d 665 (Ct. App. 1992). If a sufficient number of legislators had reached agreement that the status quo presumption should not apply when an existing custody order is for sole custody, then surely the legislature would have chosen a means less obscure than § 767.325(5m) to achieve that result. The most reasonable interpretation of the legislature's failure to specifically repeal the status quo presumption contained in § 767.325(l)(b)2 is that the legislature intended for the status quo presumption to survive its sweeping changes of the law.
*330¶ 28. The dissent's interpretation of the statutes is unworkable because it fails to provide direction to circuit courts in deciding close cases, the very cases in which a presumption is most important. The dissent's suggestion is unworkable precisely because it directs that conflicting presumptions be applied simultaneously. The dissent suggests that in close cases the circuit court's final task is to "assess the strength of the competing legislative directives on the record before it, and ultimately, to determine which should receive the greater weight in the case at hand." Dissent at ¶ 60. But this is no guidance at all. How does a circuit court "assess the strength of the competing legislative directives" in a close case? The result of adopting the dissent's approach is predictable: judges who, in general, favor sole custody will likely give that presumption greater weight. Judges with no personal preference will be left adrift with no guidance as to which presumption should be given "greater weight." Certainly the legislature did not intend this.
Equal Protection
¶ 29. Palmersheim next argues that if the status quo presumption of Wis. Stat. § 767.325(1) (b) cannot be rebutted by the joint custody presumption in Wis. Stat. § 767.24(2)(am), then application of the status quo presumption in § 767.325(1)(b) deprives him of his constitutional right to equal protection under the law. He argues that because the right to raise one's child is a fundamental right, there must be a compelling state interest in maintaining a presumption that the status quo is in a child's best interest in custody and placement modification proceedings where the initial custody and placement determinations were made prior to the 1999 legislative changes. Palmersheim claims that he and *331other similarly situated parents who were subject to the presumption of sole legal custody prior to the 1999 legislative changes are being deprived the benefit of the presumption that joint legal custody is in the child's best interest in modification proceedings by operation of § 767.325(l)(b)2. He points out that this presumption is fair for those parents seeking modification after the 1999 legislation was enacted but parents who seek to modify custody and placement where their last custody and placement orders occurred prior to 1999 will never enjoy a presumption of joint custody and maximized physical placement.
¶ 30. We review a constitutional challenge to a statute de novo. Bethke v. Lauderdale of La Crosse, Inc., 2000 WI App 107, ¶ 15, 235 Wis. 2d 103, 612 N.W.2d 332. We give no deference to the circuit court's determination in this matter. Griffin v. Milwaukee Transp. Servs., Inc., 2001 WI App 125, ¶ 4, 246 Wis. 2d 433, 630 N.W.2d 536. A statute enjoys a presumption of constitutionality and a party challenging a statute must establish its unconstitutionality beyond a reasonable doubt. Sambs v. City of Brookfield, 97 Wis. 2d 356, 370, 293 N.W.2d 504 (1980).
¶ 31. A party seeking to challenge the constitutionality of a statute on equal protection grounds must demonstrate that the statute treats similarly situated persons differently. Aicher v. Wisconsin Patients Comp. Fund, 2000 WI 98, ¶ 56, 237 Wis. 2d 99, 613 N.W.2d 849. We apply a strict scrutiny analysis where the statute impinges-upon a fundamental right or creates a classification that disadvantages a suspect class. Id.
*332¶ 32. We conclude that applying Wis. Stat. § 767.325(l)(b)2 to the facts of this case implicates a fundamental right. We recognize that parents have a fundamental right to the care and custody of their children. See Barstad v. Frazier, 118 Wis. 2d 549, 556-57, 348 N.W.2d 479 (1984). We further conclude, however, that the state's interest in protecting the best interest of a child by imposing a rebuttable presumption of maintaining the status quo in custody and physical placement modification proceedings is compelling and that § 767.325(l)(b)2 is narrowly tailored to achieve that purpose.
¶ 33. First, because we have concluded that the legislature failed to create a presumption that joint legal custody applies in custody and placement modification proceedings, all parties seeking modification of custody and placement orders, regardless of the date the last order was entered, are treated the same; the presumption that the status quo is in the child's best interest will be applied.
¶ 34. Second, the legislative decision to leave in place the presumption of continued custody and placement reveals its belief that it is inherently harmful to change a child's situation absent some change in circumstances that makes the status quo no longer in the child's best interest. Third, parents such as Palmer-sheim will not in all instances be treated differently than post-1999 legislation parents. The status quo presumption applies even for parents where the court makes an initial custody and placement determination after the 1999 reform legislation went into effect. Wisconsin Stat. § 767.24(2) provides certain circumstances under which a circuit court may order sole *333custody to a parent. Thus, a parent seeking modification of that order will he similarly affected as a pre-1999 legislation parent who carries the burden of demonstrating that it is in his or her child's best interest to modify custody or placement. In essence, continuity in custody and placement circumstances is beneficial for children, which constitutes a compelling state interest, even where the mother originally acquired custody due to the sole legal custody presumption. We conclude the circuit court applied the correct legal standard by applying Wis. Stat. § 767.325(1)(b) to the facts of this case.
CONCLUSION
¶ 35. While we agree with Palmersheim that there was a substantial change in circumstances, we conclude that the circuit court applied the correct standard of law under Wis. Stat. § 767.325(1)(b)2, a presumption that maintaining the current custody and physical placement schedule is in the child's best interest. We also conclude Palmersheim's equal protection rights have not been violated by operation of § 767.325(1)(b)2. We therefore affirm the order denying Palmersheim's request for modification of custody and physical placement.
By the Court — Order affirmed.
All references to the Wisconsin Statutes will be to the 2001-02 version unless otherwise noted.
Palmersheim argues tangentially that the circuit court erroneously exercised its discretion by not awarding him equal placement, claiming Wis. Stat. § 767.24(4) requires the court to maximize his physical placement time with his son. The heart of this case centers on whether the circuit court applied the proper legal standard in determining what was in Nicholas's best interest. Palmersheim pays scant attention to the physical placement aspect of the case; thus, our inquiry focuses on whether the circuit court erred in its application of the law to the facts of this case. In any event, the circuit court generally addressed the factors in Wis. Stat. § 767.24(5) in determining whether a change in custody and physical placement was in Nicholas's best interest. We will not disturb the circuit court's findings of fact unless the record clearly does not support them.
Wisconsin Stat. § 767.24(2)(am) reads, "The court shall presume that joint legal custody is in the best interest of the child."
Wisconsin Stat. § 767.325(5m) provides, "In all actions to modify legal custody or physical placement orders, the court shall consider the factors under s. 767.24(5) and shall make its determination in a manner consistent with s. 767.24."
We deal here with the provision governing modification motions after two years. The logical extension of Palmersheim's argument is that the joint custody presumption also trumps Wis. Stat. § 767.325(l)(a). That subsection plainly intends to foster stability for children by setting a very high bar for custody changes within two years of the original order. The moving party must, with some exceptions, show "by substantial evidence that the modification is necessary because the current custodial conditions are physically or emotionally harmful to the best interest of the child ...." Id. This status quo presump*325tion'is much stronger than the one at issue here and, under Palmersheim's approach, even more "inconsistent" with Wis. Stat. § 767.24.
The legislature is delegated the task of promulgating laws and, in the process, must do so in a rational and logical manner *328so as to avoid absurd results. The legislature neglected this task by creating a new presumption without explicitly eliminating or repealing the old one. Palmersheim strains to reconcile these conflicting presumptions by suggesting that the presumption of joint legal custody rebuts the presumption of maintaining the status quo. We cannot find any reference in the legislative history to suggest the legislature intended to maintain two statutory presumptions, with the joint legal custody presumption rebutting the status quo presumption.