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Doomsday Calculation

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The Doomsday Method Blueprint

0. Core Concept

Every year maps an anchor weekday called its Doomsday. Weekdays translate directly to numerical indices: Sun=0, Mon=1, Tue=2, Wed=3, Thu=4, Fri=5, Sat=6.

1. Century Anchors (The 0-5-3-2 Mapping)

Century anchors follow a straightforward timeline sequence of 0, 5, 3, 2 that loops indefinitely every 400 years:

Century	Anchor Day	Key Value	Logical Anchor Rule
1700s	Sunday	0	The index starts at 0
1800s	Friday	5	Steps to 5
1900s	Wednesday	3	Steps to 3
2000s	Tuesday	2	Current Century is locked to 2 ("Y2K Tuesday")
2100s	Sunday	0	The 0-5-3-2 map loop resets completely

2. Year Contribution Calculation

Take the last two digits of the year (YY) and execute the standard algorithm:

Divide YY by 12 to find a (the quotient).
Find the remainder of that division, b.
Divide b by 4 to get c (drop decimals).
Add Century Anchor + a + b + c. Simplify this sum mod 7 to track your final localized Year Doomsday.

3. Fixed Month Anchors

These benchmark days always fall directly on Doomsday:

Even Months: 4/4, 6/6, 8/8, 10/10, 12/12.
"I work 9-to-5 at the 7-11": 5/9, 9/5, 7/11, 11/7.
March: "Pi Day" (March 14).
January / February (The Leap Year Shifters):
• Common Year: Jan 3, Feb 28.
• Leap Year: Jan 4, Feb 29.

Historical Calendar Modes (COMING SOON)

1. Gregorian (Proleptic)

This mode uses the modern Gregorian calendar for all years, including historical dates before 1582 - its creation.

Leap Rule: Every 4th year is leap, except centuries not divisible by 400.

Century Pattern: The repeating 0-5-3-2 anchor cycle.

2. Historical Europe (1582 Reform)

Continental Catholic Europe switched from the Julian calendar to the Gregorian calendar in October 1582.

Calendar Split:
Julian calendar: up to Thursday, October 4, 1582.
Gregorian calendar: beginning Friday, October 15, 1582.

Missing Dates: October 5-14, 1582 never existed.

3. Historical Britain (1752 Reform)

Britain and the American colonies remained on the Julian calendar much longer than continental Europe.

Calendar Split:
Julian calendar: up to Wednesday, September 2, 1752.
Gregorian calendar: beginning Thursday, September 14, 1752.

Missing Dates: September 3-13, 1752 never existed.

4. Julian

Julian Leap Rule: Every year divisible by 4 is leap.

Julian Century Anchors: Julian century anchors move backward by 1 weekday every century. A simple mental sequence is: 1600s → 6 (Saturday)
1700s → 5 (Friday)
1800s → 4 (Thursday)
1900s → 3 (Wednesday)
2000s → 2 (Tuesday) The cycle continues backward mod 7.

Mental Calculation Notes

Gregorian dates use the standard modern 0-5-3-2 century-anchor cycle.
Julian dates use different century anchors because the Julian calendar overcounts leap years compared to Gregorian rules.