How does this calculator work?
It uses the astronomy-engine library in your browser to compute Sun and Moon positions for your latitude, longitude, and elevation. Hijri month starts follow crescent visibility rules — no data is sent to a server.
Interactive calculator loads when JavaScript is enabled. All calculations stay on your device.
Usage
- Find the Islamic date for a Gregorian birthday or civil event.
- Plan a Gregorian deadline starting from a known Hijri date.
- Convert at your city coordinates for local moon-sighting context.
- Compare Yallop crescent visibility with Umm al-Qura calendar rules.
- Review moon phase around a date for religious observance planning.
Knowledge
The Hijri (Islamic) calendar is lunar: months begin when the new crescent is deemed visible. A year has 12 months of 29 or 30 days, so Hijri dates shift about 11 days earlier each Gregorian year.
Yallop criterion. Estimates crescent visibility at sunset using astronomical data. Zones A–D are treated as visible; E and F defer the month by one day.
Umm al-Qura. A fixed rule used in Saudi Arabia from 1900 onward: the month starts when the Moon sets after the Sun at Mecca. Automatic mode picks Yallop before 1900 and Umm al-Qura from 1900 on.
Why dates differ. Communities may follow local sighting, a national calendar, or scholarly opinion — conversions can legitimately differ by one day. Choose the era rule and location that match your use.
FAQs
What is the Yallop criterion?
The Yallop (1997) method estimates whether the new crescent was visible at the best viewing time after sunset. Zones A–D count as visible; zones E and F are treated as invisible and defer the month start by one day.
What does the Automatic era rule do?
Before 1900 it applies Yallop sighting at your coordinates. From 1900 onward it uses the Umm al-Qura rule at Mecca (the Moon must set after the Sun). You can override this with the era dropdown.
Why can Hijri dates differ by one day?
Hijri dates depend on sighting location, the era rule, and whether a community uses local moon sighting or a fixed calendar. Pick the rule and coordinates that match your use case.
Which direction should I convert?
Use Gregorian → Hijri to find the Islamic date for a civil birthday or event. Use Hijri → Gregorian to plan a Gregorian deadline from a Hijri date.
Is my data stored or sent to a server?
No. All astronomy and calendar math runs locally in your browser. Nothing is uploaded or saved on a server.