---
title: "What is event time?"
description: "Event time is when something happened in the world, taken from the source's own timestamp. How it differs from ingestion time, with a backfill example."
canonical: https://past.dev/glossary/event-time
last-updated: 2026-09-02
---
# What is event time?

Source: https://past.dev/glossary/event-time

Event time is the moment something happened in the world, taken from the source record's own timestamp rather than from when a system processed it. An email sent on March 3, 2024 has an event time of March 3, 2024 even if it reaches a memory system in 2026. Systems that keep event time can place facts in the order the world produced them and answer date questions correctly.

## How event time is recorded

Every record states when its content happened: an email's `Date` header, a meeting's scheduled start, a ticket's created field, a contract's effective date. Event time is read from that statement. It is fixed by the source and does not change when the record is copied, exported, or loaded into another system.

- **Source of truth**: the record's own timestamp, kept with its timezone offset.
- **Contrast**: [ingestion time](/glossary/ingestion-time) is when a system received the record. The two can differ by years.
- **Storage**: temporal systems keep both values. Keeping only one loses either world history or auditability. See [bitemporal memory](/glossary/bitemporal-memory).

> **Rule**
>
> If a record carries no usable timestamp, store the event time as unknown instead of silently substituting the import date. The substitution misdates the fact by the length of the backlog.

## Why it matters for AI agents

An agent that dates facts by arrival believes its import order instead of the world's order. Backfilled data then reads as new, and the newest upload wins every conflict even when it describes the oldest events.

Event time makes three operations correct: [event ordering](/glossary/event-ordering), [point-in-time recall](/glossary/point-in-time-recall) of what was true on a given date, and supersession, since a 2024 statement should never replace a 2025 fact merely because it arrived later. past.dev takes an explicit timestamp with each record at ingest and dates the resulting facts by that event time.

## Event time in practice

A team connects a mailbox to a memory system on June 10, 2026. The import includes a message from the CFO sent on November 18, 2024: "The Basel office closes at the end of the year."

| Field | Value |
| --- | --- |
| Event time | November 18, 2024, the email's own Date header |
| Ingestion time | June 10, 2026, when the import ran |
| Read with event time | A 2024 announcement, old news by 2026 |
| Read without it | Dated June 2026 and treated as a fresh announcement |

See [event time vs ingestion time](/guides/event-time-vs-ingestion-time) for choosing which axis each query should use.

## Related concepts

- **[Ingestion time](/glossary/ingestion-time)**: when the system received the record, the other half of a bitemporal pair.
- **[Bitemporal memory](/glossary/bitemporal-memory)**: storage that keeps event time and ingestion time side by side.
- **[Event ordering](/glossary/event-ordering)**: sequencing stored events by event time rather than arrival.
- **[Validity window](/glossary/validity-window)**: the dated span during which a fact was true, anchored at event time.
- **[Point-in-time recall](/glossary/point-in-time-recall)**: querying what was true as of a date.

## Frequently asked questions

### What is the difference between event time and ingestion time?

Event time is when something happened in the world, read from the source's own timestamp. Ingestion time is when a system received the record. A 2024 email imported in 2026 has a 2024 event time and a 2026 ingestion time.

### Why does event time matter for AI agents?

Agents that date facts by arrival order misread backfilled data as new and resolve conflicts in favor of whatever was imported last. Event time lets an agent order events, answer date questions, and supersede facts correctly.

### Where does event time come from?

It comes from the record itself: an email's date header, a meeting's scheduled start, a document's stated effective date. The system stores it as given rather than substituting its own clock.

## Related

- [Ingestion time](https://past.dev/glossary/ingestion-time)
- [Bitemporal memory](https://past.dev/glossary/bitemporal-memory)
- [Event time vs ingestion time](https://past.dev/guides/event-time-vs-ingestion-time)
- [Memory API quickstart](https://past.dev/docs/memory-api/quickstart)
- [Benchmark methodology](https://past.dev/benchmarks/methodology)