Hanke-Henry Permanent Calendar
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The Hanke-Henry Permanent Calendar 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. 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 extra 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.
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. 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  of Judaism and Christianity. This calendar also urges its own adoption and attempts to convince people it is the best calendar ever proposed. Henry tells supporters of other calendar proposals to support a switch to the C&T.
In 2004, Dick Henry, a professor of astronomy at Johns Hopkins University, proposed the adoption of a calendar known as Common-Civil-Calendar-and-Time, which he described as a modification to a proposal by Robert McClenon. This version had essentially the same structure given above, but inserted its leap week named "Newton" between June and July. The leap rule was chosen to match the ISO week leap rule, to minimise the variation in the start of the year relative to the Gregorian calendar.
He had advocated transition to the calendar on 1 January 2006 as that is a year in which his calendar and the Gregorian calendar begin the year on the same day. After that date passed, he recommended dropping off 31 December 2006 to start in 2007, or dropping 30 and 31 December 2007 to start 2008.
In late 2011 the calendar was revised by Johns Hopkins economist Steve Hanke into its current form, by moving the leap week from the middle to the end of the year and renaming it "Xtra", producing the Hanke-Henry Permanent Calendar. The adoption process officially began on 1 January 2012 and the current target date for universal adoption is 1 January 2017, according to the calendar's website.
The key difference between Robert McClenon's calendar proposal and Henry's modification is that it has a simple rule for determining which years have a leap week. This rule resembles the Gregorian leap year rule. Years whose numbers are divisible by 5 have a leap week, but years whose numbers are divisible by 40 do not have a leap week unless also divisible by 400. The main drawback of this rule is that the new year varies 17 days relative to the Gregorian new year (e.g. year 1965 begins 11 days earlier than Gregorian 1965 and year 2036 begins 6 days later than Gregorian 2036), whereas Henry's rule ensures that the new year always begins within four days of the Gregorian new year.
- Holidays such as Christmas and New Year's Day as well as birthdays always occur on the same day of the week every year.
- The calendar itself is permanent, it does not change year to year, with the exception of the need to add a week at the end of every 5 or 6 years.
- Quarters all have the same number of days simplifying financial calculations. This calendar would also have prevented Apple’s Q4 2012 reporting fiasco. 
- Unlike many other reform proposals, it does not change the days of the week.
- The calendar starts on the same day every year, Sunday, the 1st of January.
- As in the Gregorian calendar, Sunday to Sunday is always seven days, as is Saturday to Saturday, or Friday to Friday. Because no days are ever added outside a seven-day week, there should be no objection from religious groups concerned about weekly holy days. (In proposals that add single days outside the week, a true "seventh day" of rest or worship would drift between weekends and weekdays.)
- Not as precisely aligned with the solar year as the existing Gregorian calendar and some proposed reform calendars.
- Requires continued use of the Gregorian calendar for certain agricultural purposes.
- All computer date-handling will have to be fixed, which will be much more complicated than the Y2K fix.
- US-biased, not compatible with international standards, such as ISO 8601, which start the week on Monday, hence also the week year. This issue could be resolved by modifying the calendar to begin in a year where 1 January falls on a Monday, instead of a Sunday. e.g. Monday, 1 January 2018 instead of Sunday, 1 January 2012.
- Leap years are more difficult to determine than in some other proposals, being essentially dependent on the Gregorian calendar's weekday cycle.
- There is no 31st of October, removing Halloween from the current date.
Other calendar reform proposals
- Leap week calendar Week added at the end of certain years.
- The World Calendar A 12-month, perennial calendar with equal quarters proposed by Elisabeth Achelis in the 1930s.
- International Fixed Calendar A perennial calendar with 13 months of 28 days each, with one day at the end of each year belonging to no month or week. It is also known as the Cotsworth plan and the Eastman plan.
- "The Hanke-Henry Date and Time, Everywhere in the World"
- "Is It Time to Overhaul the Calendar?", by Stephanie Pappas, Scientific American.
- The Fourth Commandment at the CCC&T Calendar website.
- "What if We MISS 2006 January 1 Sunday?"
- , GlobeAsia, 2013
- Hanke-Henry Permanent Calendar Proposal
- Johns Hopkins press release on C&T
- Slashdot discussion of Dick Henry's C&T
- "Is it time to overhaul the calendar? Profs have a plan", by Stephanie Pappas, LiveScience Senior Writer, 28 December 2011
- "Proposed calendar would make it same exact day, next year", By Elizabeth Weise, USA Today, 29 December 2011
- "Changing Times" by Steve H. Hanke and Richard Conn Henry, Cato Institute
- "Proposed New Calendar Would Make Time Rational", By Brandon Keim, 28 December 2011
- Press Release: "Time for a Change? Johns Hopkins Scholars Say Calendar Needs Serious Overhaul"
- "Taking Calendar Reform Viral" by Steve H. Hanke, Cato Institute