A computationally cheap method is described for estimating monthly means of climatological variables on alternate calendars, given their monthly means on an original calendar. This can be used to convert archived paleoclimatic GCM monthly mean results originally saved on the modern calendar to a more appropriate equi-angular calendar, thus avoiding spurious orbital signals when comparing monthly means for modern and past simulations. The method is tested using GCM simulations for the present and 126 ka BP. The method works well for temperature, which has relatively smooth seasonal cycles, and satisfactorily for other climatological variables in most regions except mid to high southern latitudes. It is also satisfactory for precipitation in most regions, but in the tropical ITCZ, it fails to significantly reduce the spurious calendar errors. However, this is largely due to high-frequency natural interannual variability in the 10-year simulations, and the errors are of the same order as the uncertainty in monthly means on a single calendar.