Last updated 25 October 2026
How Harmonic works
Thirteen equal months. Four complete weeks in each. A Leap Week keeps the seven-day cycle continuous.
A predictable month
Harmonic Calendar is a variant inspired by the International Fixed Calendar. Each of its 13 months has 28 days: four full weeks, starting on Monday and ending on Sunday. The same day number always falls on the same weekday.
The months are January, February, March, April, May, June, Sol, July, August, September, October, November and December. Sol sits between June and July.
A full week for the correction
Thirteen months make 364 days. Harmonic adds a seven-day Leap Week after December in 71 of every 400 years. A common year has 364 days; a leap-week year has 371. The 400-year total is 146,097 days, matching the Gregorian calendar’s cycle while keeping every week intact.
This differs from the standard International Fixed Calendar, which uses separate Year and Leap Days outside its weekly cycle. Harmonic uses Monday-first months and a Leap Week instead.
Compare the same day
Gregorian January 1, 2001 is Harmonic January 1, 2001, a Monday. From that shared starting point, the calendars follow different month and leap rules, so month and year boundaries can differ. Both views use your device’s local time zone.
Use H/G to switch the selected day between calendars. Open the month grid to browse months and years, select a day, or return to Today. Month browsing supports years 1–9999.
What the app is for
Harmonic Calendar is a date viewer for exploring an alternative calendar and keeping today visible. It does not manage events or reminders, connect to calendar accounts, or change your device’s system calendar. Use Gregorian dates for appointments, deadlines and official dates.
Need help? Visit support. Read how the app handles information in the privacy policy.