The basic difficulty is that every year has 52 weeks of 7 days plus one day or, in a leap year, 2 days. The calendar contains 28 different kinds of months, with 28, 29, 30 or 31 days, starting on any one of 7 weekdays, and anything from 24 to 27 working days. Every year shifts and is different from the preceding and following years, so that comparability is difficult and inaccurate. Weekdays and month dates never agree with an incoming or outgoing year. Equal divisions of quarters and half-years are lacking. Considerable amounts of time and money are allocated each year to compiling new calendars and schedules because of this variability. Holidays falling on fixed dates which vary in their position within the week give rise to different conditions each year. Statistics for fixed periods have to be corrected for such variability to make them comparable between months and years. Variable church holy days are a continuous concern to both priest and layman. Law courts encounter many difficulties to reconcile wandering weekdays and holidays, resorting to such expressions as 'the first Tuesday after the first Monday in November'.
Other calendars continue to exist in parallel with that based on the Christian-oriented system. These include the Islamic, Hindu and Chinese calendars. The most radical attempt at redirecting the temporal identity of an entire culture through calendrical manipulation, in order to eliminate the religious bias, was that during the French Revolution.