Ichino and Moretti (2009) find that menstruation may contribute to gender gaps in absenteeism and earnings, based on evidence that absences of young female Italian bank employees follow a 28-day cycle. We find this evidence is not robust to the correction of coding errors or small changes in specification, and we find no evidence of increased female absenteeism on 28-day cycles in data on school teachers. We show that five day work weeks can cause misleading group differences in absence hazards at multiples of seven, including 28 days, and illustrate this problem by comparing absence patterns of younger males to older males. (Contains 1 figure, 3 tables and 21 footnotes.)