13 Month Calendar Date Converter

Convert any Gregorian date to its 13 month calendar equivalent — and back again.

What this tool does

The 13 month calendar divides the year into 13 equal months of 28 days each, plus a Year Day (and a Leap Day every four years). This converter takes any Gregorian date — past, present, or future — and tells you exactly where it falls in the 13 month calendar: which of the 13 months, which day of the month, and which perpetual weekday.

It also works in reverse: enter a 13 month calendar date and see the Gregorian equivalent. Because the 13 month calendar is perpetual, the same date always falls on the same weekday — every year, forever.

How the 13 month calendar conversion works

Each 13 month calendar year begins on January 1st (same as Gregorian) and runs through 13 months of exactly 28 days: January, February, March, April, May, June, Sol, July, August, September, October, November, December. Sol is the 13th month, inserted between June and July. After December 28th comes Year Day (December 29th in Gregorian terms) — a single day belonging to no month and no week. In leap years, a Leap Day is added between June 28th and Sol 1st.

To convert: count the day of the year (1–365 or 366), then divide by 28. The quotient gives the month index, and the remainder gives the day of the month. Year Day and Leap Day are handled as special cases. See the Sol month explainer and 13 month calendar vs Gregorian for the full mechanics.

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