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13-Month Calendar
13 months × 28 days = 364 days
+ Zero Day = 365 days
Your 13-Month Birthday
Enter your Gregorian birth date to discover your fixed calendar birthday — it falls on the same weekday every year, forever.
Discover My 13-Month Birthday
Your IFC Birthday
Enter your birth date above to see which day of the 13-month calendar you were born on — and the weekday it falls on forever.
Appearance
Customize your calendar display and features.
Dark Mode
Using light theme
Event Countdown
Countdown badges hidden
Moon Phases
Moon phases hidden
Dreamspell (13 Moon, 1990s system)
Off. Reads the Gregorian date and does not move the fixed calendar year.
Show Holidays
Display public holidays on calendar
Select your region to see local holidays
Choose a preset or tap any month to customize.
Import
Restore a backup, or import an .ics file or a Google Calendar .zip. Imported events stay on this device.
Import
From backup or calendar file
JSON Backup
ThirteenCal backup file
ICS Calendar
.ics or Google Calendar .zip
Subscribed calendars
Add a webcal, http, or https feed. Events stay read-only and refresh when you open the app if the feed is more than 6 hours old.
Sign in to load feeds on the web.
IFC → Gregorian Mapping
2026 date ranges
Key Differences
IFC vs Gregorian
Date Diagnostics
IFC ↔ Gregorian round-trip
About
International Fixed Calendar variant
Every month starts on Sunday
Zero Day and Leap Day sit outside every week