The distance between the birth date and the school entry cutoff has been repeatedly used as an exogenous instrument to examine the impact of several educational programmes. In this work, we analyse the validity of this instrument for the case of Argentina. Considering multiple waves of the Permanent Household Survey we detect the existence of discontinuities in the distribution of births around the school entry cutoff (30 June). These discontinuities suggest that parents act strategically. In particular, they defer birth dates to days after the cutoff. This effect is especially large considering a bandwidth of 7 days and in boys.