13 Month Calendar Calculator

Here’s a table summarizing the key information you need to know about a 13-month calendar:

Feature	Description
Number of Months	13
Days per Month	28 (4 weeks exactly)
Total Days	364 (plus 1 or 2 extra days)
Extra Month Name	“Sol” (typically placed between June and July)
Extra Day(s)	“Year Day” (and “Leap Day” in leap years)
Weekday Consistency	Each date falls on the same day of the week every year
Month Start	Every month starts on a Monday
Month End	Every month ends on a Sunday
Quarters	Each quarter has 91 days (3 months of 28 days each)
Benefits	Easier financial comparisons, consistent scheduling
Challenges	Adoption, software updates, cultural adjustments
Historical Usage	Kodak used it from 1924 to 1989
Origin	Proposed by Moses B. Cotsworth in 1902
Popular Advocate	George Eastman (Kodak founder)

Key points to remember:

1. The calendar provides consistency, with each date always falling on the same weekday16.
2. It simplifies financial calculations and comparisons across periods12.
3. The extra month “Sol” is typically placed between June and July6.
4. The “Year Day” (and “Leap Day” in leap years) are often placed outside the regular month structure6.
5. While it offers benefits for business and scheduling, widespread adoption faces significant challenges35.
6. The concept has been around since the 18th century, with various iterations proposed over time6.

This table provides a comprehensive overview of the 13-month calendar concept, its structure, benefits, and historical context.