Technical Abstract: The calibration of basin-scale hydrologic models consists of adjusting parameters such that simulated values closely match observed values. However, due to inevitable inaccuracies in models and model inputs, simulated response hydrographs for multi-year calibrations will not be perfectly synchronized with observed response hydrographs at the daily time step. An analytically derived formula suggests that when timing errors are significant, traditional calibration approaches may generally underestimate the total event flow volume. An event-adaptive time series is developed and incorporated into the Nash-Sutcliffe Efficiency (NSE) objective function to diagnose the potential impact of event flow synchronization errors. Test sites are the 50 km**2 subwatershed l of the Little River Experimental Watershed (LREWswl) in southeastern Georgia, USA, and the 610 km**2 Little Washita River Experimental Watershed (LWREW) in southwestern Oklahoma, USA, with the Soil and Water Assessment Tool (SWAT) used as the hydrologic model. Results suggest that simulated surface runoff generation is 55% less for LREWswl when the daily time series is used compared to when the event-adaptive technique is used. Event flow generation may also be underestimated for LWREW, but to a lesser extent than it may be for LREWswl, due to a larger portion of the event flow being lateral flow.