What does estimated mean?
When we do not have a confirmed local moon-sighting result or an announcement from a trusted authority, the Hijri date is shown as an estimated calculated date. It is useful for planning and search, but it should not replace local guidance for worship, travel, or community arrangements.
The default method
Online Hijri currently uses a deterministic tabular Islamic calendar as the default method. This keeps month pages and event pages stable between the server and the browser, and avoids hidden differences caused by runtime-specific calendar data.
Why dates may differ by one day
A Hijri day begins at sunset, and the start of a Hijri month may depend on local moon sighting or an institutional announcement. Ramadan or Eid can therefore begin one day earlier or later in different countries and communities.
Umm al-Qura and other calendars
Umm al-Qura, civil calendars, tabular calendars, and local confirmed results are different methods, not different names for the same result. When a page names a method, the displayed date belongs to that method and should not be treated as universal for every region.
When to verify locally
Always check a trusted mosque or local authority before fasting, Eid, Hajj-related planning, or travel. This site uses calculated dates for planning and labels date status so estimates are not presented as official confirmation.