Sidang Isbat Corrections

The published Kemenag calendar is produced from Neo MABIMS calculations, and for years its results have always been confirmed at the Sidang Isbat. In principle, though, the month start is decided on the night of the Sidang Isbat (the rukyat on day 29), not on the printed calendar. If that decision differs from the published one, the API follows the official decision — and says so, loudly.

Why a difference can happen

The published calendar is a criteria prediction; the Sidang Isbat decision is what binds. The difference is at most ±1 day and it affects two months at once:

• the month whose start was published and overridden: its length changes 29↔30 days (the substance of the Isbat decision);
• the following months: their starts stay exactly as published until their own Sidang Isbat resolves them.

What you will see

Every response touching the corrected month (or the month before it) carries a warning:

{
  "warnings": [
    "kemenag_override: 1 Syawal 1447 H official 2026-03-20 (Sidang Isbat; published Kemenag calendar: 2026-03-21, delta -1 days)."
  ]
}

source remains mabims — the same data, just corrected.

/meta exposes the correction history, two slim fields per correction:

{
  "table_version": "1-a41d3c9be021",
  "divergences": [
    { "hijri_month": "1447-10", "delta_days": -1 }
  ]
}

What does NOT change

• The Neo MABIMS criteria results (mabims-computed, hilal cards) remain astronomically correct — the Isbat correction only reports the official decision in warnings, it does not alter the astronomy.
• Response shapes, error codes and endpoints are unchanged. Fields and warnings are additive.

Client integration recipe

1. Poll /meta once a day (5-minute cache, cheap).
2. Compare table_version with the one you stored. "none" means no correction has ever been applied.
3. When it changes: read divergences[], then re-fetch the dates falling in the corrected month and the month before it.
4. Alternatively: catch the kemenag_override: prefix on any response’s warnings[] and surface it to your users.

History

So far no correction has ever been applied — divergences[] is empty and table_version is "none". This page will be updated if a Sidang Isbat ever corrects the published calendar.

Implementation details live in SIDANG-ISBAT-FLIP.md in the repository.