This sidereal computation avoids having to do leap year adjustments , but the number of days in any given month can vary by one or two days and conversion of dates to Gregorian or day of the week computations requires one to consult the ephemeris .
This sidereal computation avoids fixed leap year rules , but the number of days in any given month can vary by one or two days . 
 Conversion of dates to the Gregorian calendar , or computing the day of the week , requires one to consult the ephemeris .