This quasi-experimental study strives to ascertain whether the change from a public school traditional calendar to a modified or year-round calendar effects academic achievement in English and mathematics. The twelve public elementary schools, not U.S. Department of Defense schools, in the core of this research are identified only as serving students of military families, stationed on the Island of Oahu, in the State of Hawai'i. Most students attending the chosen schools live in or live adjacent to military housing communities. These twelve schools were selected for their almost common socio-economic status and similar community living characteristics. Five of the elementary schools, the control group, were operating on a traditional public school calendar and the remaining seven schools, the treatment group, had transitioned to a modified calendar. Participants in the study numbered 1,389 fourth and fifth grade students and 1,458 third and fourth grade students. The amount of class days for both calendars remained the same, but were rearranged for the modified calendar schools. The modified calendar students had a shorter summer break, a longer winter and spring break, and a new one week vacation in October. In order to determine if the new modified calendar had a larger effect on academic achievement than the traditional calendar, the 2006 scores of the SAT9 English and mathematics segments of the annual Hawaii State Assessment, for all twelve schools, were analyzed. Since I was using existing scores from a database, the design chosen for this research was regression-discontinuity (RD). According to Thislethwaite and Campbell (1960), this design is a legitimate alternative to randomized experiments. The analysis of the findings indicated that the modified calendar school students in both grade 3-4 comparisons and grade 4-5 comparisons were all positive. However, only the test for grade 3-4 mathematics was significant. It should be noted that a much larger school sample might increase the statistical significance of all four comparisons. The findings of this research agree with my hypothesis, that the change to a modified calendar should positively affect achievement in English and mathematics at selected schools in Hawaii. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]