---
title: "Separate when it changed from when you learned of it."
description: "Separate when something was true from when the system learned about it."
updated: "27 August 2026"
candidate_route: "/docs/harbinger/guides/read-a-change"
---

# Separate when it changed from when you learned of it.

Separate when something was true from when the system learned about it.

## Keep effective time and observation time distinct

1. **Effective time** — When the state or relationship is asserted to hold in the real world.
2. **Observation time** — When HARBINGER received or recorded the evidence.

## Do not collapse operational states

- NO_CHANGE — a valid comparison found no supported difference.
- NO_DATA — permitted comparison input is absent.
- UNKNOWN — available evidence cannot establish the state.
- STALE — evidence exceeds the freshness required by the inquiry.
- ACCESS_DENIED — the current user cannot view the result.

## Corrections extend the record

A correction or withdrawal should not erase what was previously observed. Preserve the earlier claim, the later change, and any downstream result that depended on it.

---

Published route: /docs/harbinger/guides/read-a-change