This article reports on elementary students' understanding of time in the context of common classroom manipulatives and notational systems. Students in Grades 2 (n = 72) and 4 (n = 72) participated in problem-solving interviews involving different clocks. Quantitative results revealed that students' performances were significantly different as a function of the tool available. Descriptive case studies of 3 Grade 4 students are presented in which students demonstrated competencies in conventions related to benchmark numeric conversions between hours and Minutes and counting by 5s around the clock, yet only partial competencies related to the integral relationship between hours and minutes. Implications for theory and the treatment of time in curriculum and instruction are discussed.