This calendar is built on the "Fateh" astronomical calculation, a luni-solar system based on the 19-year Metonic cycle (19 lunar years = 235 lunar months). Seven years out of nineteen count 13 months instead of 12, through an intercalary month, "Nassii'", inserted between Rabi' al-Awwal and Rabi' al-Thani. This periodic correction keeps the start of the year anchored near spring, instead of drifting through the seasons as in the usual purely lunar Hijri calendar.

The month of Nassii' was reinstated here on the basis that the Quran does not speak of abolishing the principle of intercalation itself, but condemns, in verse 37 of Surat At-Tawba, its political misuse: postponing or advancing the sacred months according to the interest of the moment, to declare war or a truce.