Account
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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
Equinox New Year
Year starts in January
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 from a backup or import from an external calendar.
Import
From backup or calendar file
JSON Backup
ThirteenCal backup file
ICS Calendar
Google, Apple, Outlook, etc.
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: 365th day, outside any week