Important educational policy decisions, like whether to shorten or extend the school year, often require accurate estimates of how much students learn during the year. Yet, related research relies on a mostly untested assumption: that growth in achievement is linear throughout the entire school year. We examine this assumption using a dataset containing math and reading test scores for over seven million students in kindergarten through 8th grade across the fall, winter, and spring of the 2017-18 school year. Our results indicate that assuming linear within-year growth is often not justified, particularly in reading. Implications for investments in extending the school year, summer learning loss, and the development of racial/ethnic achievement gaps are discussed.