—Order, Family Court, New York County (George Jurow, J.), entered November 2, 1992, which granted the petition of the New York State Division for Youth to extend appellant’s placement with the Division for Youth and, upon the parties’ agreement, extended his placement for eight months, reversed, on the law, without costs or disbursements, and the motion denied.
On August 30, 1991, appellant first appeared in the Family Court on a juvenile delinquency petition charging him with having committed an act, which, if committed by an adult, would constitute criminal possession of a weapon in the fourth degree, and was remanded to a non-secure detention facility. On September 4, 1991, he admitted to the charge. He was continued on remand, and, on consent, a dispositional order placing him with the Division for Youth at St. Mary’s Children and Family Services for a twelve-month period was entered. No finding was made at the time that diminishing *372the period of placement by the time already spent on remand "would not serve the needs and best interests of [appellant] or the need for protection of the community” (Family Ct Act § 353.3 [5]). On August 28, 1992, the Division for Youth petitioned for an extension of placement, which appellant, through his Law Guardian, opposed as untimely under Family Court Act § 355.3 (1). The court granted the petition. We reverse.
Family Court Act § 355.3 (1) requires that a petition to extend the placement of a juvenile delinquent be filed "at least sixty days prior to the expiration of the period of placement, except for good cause shown.” Failure to do so mandates dismissal of the petition. (Family Ct Act § 355.3 [2].) Pursuant to Family Court Act § 353.3 (5), the initial period of placement is to be "credited with and diminished by the amount of time spent by the respondent in detention prior to the commencement of the placement.” The statutory intent is clear—to credit automatically the juvenile’s period of placement with pre-order-of-disposition detention time. Absent a specific finding that such credit would not serve the interests of the juvenile or the community, the credit for time in predisposition detention automatically accrues. (Cf, Matter of Mack M., 175 AD2d 869.) Since appellant’s original placement was for twelve months and he had been in detention from August 30, 1991, his placement would have expired on August 29, 1992. Thus, in the absence of "good cause”, as to which there had been no showing, the petition, not filed until August 28, 1992, was untimely and should have been dismissed. The failure to calculate correctly the expiration of a juvenile’s period of placement cannot be considered good cause.
The argument that the Division for Youth, the petitioner in seeking an extension of placement and in whose custody appellant had been placed, was not responsible for calculating the period of placement is without merit. That the statute provides for the automatic accrual of the credit is to us the clearest indication that the calculation must be made by the Division for Youth, which is in the best position to know precisely how much time must be credited to someone in its custody. The failure to perform a routine calculation cannot constitute good cause. (See, Matter of Changa W., 123 AD2d 435; Matter of Susan F, 59 AD2d 783; see also, Sobie, Practice Commentary, McKinney’s Cons Laws of NY, Book 29A, Family Ct Act § 355.3, at 575.) The argument that appellant should not be credited with the time he was absent without leave from the Division for Youth is not well taken. Since *373appellant was absent from the Division for Youth facility for only three days prior to August 29, 1992 (August 17th to August 20th), the normal expiration date of his placement would have been September 1, 1992. The petition to extend placement was, as noted, filed on August 28, 1992, at least 57 days late.
A second ground on which to dismiss the petition to extend placement exists since the decision thereon was "not rendered within the period of placement or any temporary extension thereof’ (Family Ct Act § 355.3 [5]). In all, two temporary extensions encompassing 45 days may be granted. (Ibid.) Once the original placement expires, the Family Court loses jurisdiction over the juvenile unless an extension is timely sought and the determination made before the expiration of placement or within the period of a temporary extension of placement. (People ex rel. Margolis v Dunston, 174 AD2d 516, 517, lv denied 79 NY2d 753.) Although the petition to extend placement was filed on August 28, 1992, appellant’s placement was never extended. Thus, with no order in existence temporarily extending appellant’s placement, the Family Court, on November 2, 1992, lacked the jurisdiction to extend his placement. Concur—Sullivan, J. P., Rosenberger and Asch, JJ.