Hanke-Henry Permanent Calendar

The Hanke–Henry Permanent Calendar (HHPC) is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars which maintain synchronization with the solar year by intercalating entire weeks rather than single days. It is a modification of a previous proposal, Common-Civil-Calendar-and-Time (CCC&T). With the Hanke–Henry Permanent Calendar, every calendar date always falls on the same day of the week.

While many calendar reforms aim to make the calendar more accurate, the Hanke–Henry Permanent Calendar focuses on making the calendar perennial, so that every date falls on the same day of the week, year after year. The familiar drift of weekdays with respect to dates results from the fact that the number of days in a physical year (one full orbit of earth around the sun, approximately 365.24 days) is not a multiple of seven. By reducing common years to 364 days (52 weeks), and adding an extra week every five or six years, the Hanke–Henry Permanent Calendar eliminates weekday drift and synchronizes the calendar year with the seasonal change as the Earth circles the Sun. The leap week, or "mini-month", known as "Xtr (or Extra)", occurs every year that either begins or ends in a Thursday on the corresponding Gregorian calendar, and falls between the end of December and the beginning of January. Thus, each year always begins between December 28 and January 3 in the Gregorian calendar. This is effectively the same rule as in ISO week dates.

Under the Hanke–Henry Permanent Calendar January, February, April, May, July, August, October and November have thirty days, while March, June, September, and December have thirty-one, so that each quarter contains two 30-day months followed by one month of 31 days (30:30:31). While the Hanke–Henry Permanent Calendar changes the length of the months, the week and days remain the same. As part of the calendar proposal, time zones would be eliminated and replaced with UTC.

Henry argues that his proposal will succeed where others have failed because it keeps the weekly cycle perfectly intact and therefore respects the Fourth Commandment (Remember the sabbath day, to keep it holy) of Judaism and Christianity.

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