Comparison of Earth Calendars

Name Type Perpetual months/year days/month days/week Status Pros * Cons *
Earthian solar no 12 30-31 7 proposed
  • Uses simple, easy-to-learn patterns and rules.
  • One simple and accurate leap year rule, with a short cycle of only 33 years.
  • Calendar year begins on northern vernal equinox.
  • Months closely aligned with seasonal markers.
  • Calendar year can be accurately divided by 2, 3, 4, 6, or 12 on month boundaries.
Gregorian solar no 12 28-31 7 current
  • Widely used. Most people know it.
  • Overly variable month lengths with inconsistent pattern.
  • Months not aligned with either seasonal markers or lunar cycle.
  • Slightly complex leap year rules that cause calendar year to drift from seasonal markers over a century.
Traditional Chinese lunisolar no 12-13 29-30 7 current
  • Months equal to lunations.
  • Complex intercalation rules.
  • Includes both lunisolar and solar components.
  • 13-month years cannot be easily divided by 2, 3 or 4.
Hebrew lunisolar no 12-13 29-30 7 current
  • Months equal to lunations.
  • Uses rules for month intercalation (based on the Metonic cycle) rather than lunar observation, which is better for computer programs.
  • Inconsistent pattern of month lengths.
  • 13-month years cannot be easily divided by 2, 3 or 4.
Islamic lunar no 12 29-30 7 current
  • Months equal to lunations.
  • No month length rules. Months determined by lunar observation, which is a problem for computer programs.
  • Calendar year is 11 days shorter than the tropical year, hence seasons occur later each year.
Iranian solar no 12 29-31 7 current
  • Calendar year begins on northern vernal equinox.
  • Months closely aligned with seasonal markers.
  • First 6 months 31 days; second 6 months 30 days. Year cannot be accurately divided into equal parts on month boundaries.
  • No leap year rules; year beginning determined by northern vernal equinox, which is a problem for computer programs.
Indian National solar no 12 30-31 7 current
  • Calendar year begins on northern vernal equinox.
  • Months closely aligned with seasonal markers.
  • First 6 months 31 days; second 6 months 30 days. Year cannot be accurately divided into equal parts on month boundaries.
Buddhist lunisolar no 12-13 29-30 7 current
  • Months equal to lunations.
  • Average year length longer than tropical year.
  • 13-month years cannot be easily divided by 2, 3 or 4.
Julian solar no 12 28-31 7 deprecated
  • Highly variable month lengths with no consistent pattern.
  • Average year length slightly longer than tropical year.
  • Months not aligned with either seasonal markers or lunar cycle.
Egyptian, French Republican solar yes 12 + 5-6 days 30 10 deprecated
  • Equal month lengths (not counting 5-6 surplus days).
  • Decimal weeks.
  • 5-6 day "mini-month" requires special handling.
World solar yes 12 + 1-2 days 30-31 7 proposed
  • Equal quarter lengths (not counting 1-2 surplus days).
  • 1-2 days per year outside of months require special notation.
  • Perpetual, but with varying month pages.
Positivist solar yes 13 + 1-2 days 28 7 proposed
  • Equal month lengths (not counting 1-2 surplus days).
  • 1-2 days per year outside of months require special notation.
  • 13-month years cannot be easily divided by 2, 3 or 4.
New Earth solar yes 13 28 7 proposed
  • Equal month lengths (not counting intercalary week).
  • Intercalary week embiggens leap baby problem.
  • 13-month years cannot be easily divided by 2, 3 or 4.

* There are a few advantages and disadvantages common to certain types of calendars, so in the interest of avoiding repeating myself I have listed them below instead of in the "Pros" and "Cons" columns:

Pros:

  1. All solar calendars are synchronised with the seasonal cycle.
  2. All lunar and lunisolar calendars are synchronised with the lunar cycle.
  3. In perpetual calendars dates always map to the same day of the week, regardless of the year, which can be very convenient. Because the same calendar can be used every year, this saves paper and is therefore good for the environment.

Cons:

  1. Months in solar calendars are not aligned with the lunar cycle.
  2. In lunar and lunisolar calendars, calendar years are not aligned with seasonal markers. The year length also varies in length by up to 30 days (354 - 384 days), thus causing a large leap baby problem.
  3. Non-perpetual calendars require many different page layouts for calendar months. The Gregorian Calendar is probably the worst, with 28 possible page layouts. This is less tidy or convenient, plus it wastes paper. (It is good for the printing business, however.)