We study the origins and economic consequences of conflicts between timing institutions—rules that set common schedules—and local economic activity. We focus on school–harvest conflicts by assembling a global dataset of countries’ school calendars and comparing them with the timing of harvesting. We document systematic calendar overlap, particularly in southern-latitude countries and former colonies. Instrumenting overlap with colonial variation in calendar timing, we find that a one-percentage-point increase in school–harvest overlap raises school dropout by 0.6–0.8 percentage points. Beyond colonial inheritance, similar timing conflicts can arise through domestic standardization when a uniform calendar is imposed despite local crop variation. We study this distinct channel using a Colombian reform that shifted public schools to a common national calendar, increasing overlap in some schools while reducing it in others. Using administrative census data covering the universe of rural public schools, we find that a one-percentage-point increase in school–harvest overlap raises the dropout rate by about 0.7 percentage points, while outcomes improve where overlap falls. Our estimates are robust across alternative estimators, control groups, clustering levels, and treatment definitions. Employing nationally representative Colombian household survey data, we find agricultural work among children rises in affected municipalities, consistent with a school–labor time-allocation mechanism. These findings identify institutional timing as an important determinant of human capital accumulation in agrarian economies.