What employee with the right to choose their own working hours has not felt the silent weight of critical reproach from their colleagues or their boss when they arrive later than most at work? Although this is a form of social control that seems widespread in the world of French flexitime, it is a subject that has so far received very little scholarly attention in research on day-today tempo-ralities. Yet in the era of flexible working hours, the persistence of daily peak commuting periods could be partly explained by the presence of implicit time-keeping norms in the workplace. In this article, therefore, we try to demonstrate the existence of 'social norms on working hours' that continue to put pressure on flexitime workers to arrive at the office at the same time as their colleagues. Drawing on a field survey conducted in big companies in the Paris region, we show that there exist two types of social norms on working hours that directly contribute to morning peak-time transport congestion: 'the norm of the disciplined worker' and the 'norm of the dedicated executive'.