Hanke–Henry Permanent Calendar

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The Hanke–Henry Permanent Calendar (HHPC) is a proposal to reform the calendar so every date falls on the same day of the week every year. It keeps the weekly cycle intact, and it would also replace time zones with a single standard time (UTC).

How it works
- Most years have 364 days (52 weeks). Every so often, an extra leap week is added so the calendar stays in sync with the seasons.
- The leap week, called Xtra or Extra, happens in years when the corresponding Gregorian year starts or ends on a Thursday. The leap week sits between the end of December and the start of January.
- Because of this rule, the year in the Gregorian calendar always starts between December 29 and January 4.

Month lengths
- The calendar changes how long each month is, but not how long the week is. January, February, April, May, July, August, October, and November have 30 days. March, June, September, and December have 31 days. Each quarter then follows a 30-30-31 pattern.

Dates and anniversaries
- The weekday for every date never drifts, making anniversaries easier to remember. Some fixed-date observances would move to a nearby day, or celebrations could use a method like “the 30th or the last day of the month,” or compare to the original year for historical events.

Time zones
- Time zones would be dropped in favor of a single UTC time standard.

Origins and evolution
- The idea began as Common-Civil-Calendar-and-Time (CCC&T) in 2004, building on earlier proposals. The leap week was moved to the end of the year in 2011 and renamed Extra, giving the Hanke–Henry Permanent Calendar.
- Proponents aimed for a near-term start date and later aligned the calendar with international standards that begin the week on Monday.
- A calendar app to help convert between the Gregorian and HHPC appeared in 2016 but is no longer available as of 2018.
- In 2019, a proposal suggested promoting the calendar politically in the United States, even joking about naming the leap week after a president.

Other calendar ideas
- Other proposals include Symmetry010 and Symmetry454, which experiment with different month lengths and patterns. Older ideas like Pax Calendar and the International Fixed Calendar also use leap weeks or extra days, but HHPC stands out for keeping a regular week and a simple leap rule aimed at a perpetual, week-aligned year.