> For AI agents: the complete documentation index is available at https://open-spaced-repetition.github.io/ts-fsrs/llms.txt, the full documentation bundle is available at https://open-spaced-repetition.github.io/ts-fsrs/llms-full.txt.

# Date

`dateChrono` is the preset used by `DefaultScheduler`. Its time schema accepts valid `Date` objects, so convert stored strings or timestamps to `Date` before calling the scheduler. The current schema also rejects `new Date(0)` (the exact Unix epoch) and invalid dates.

Configure Date with `fractionalDays` at the top level of the scheduler config (default `false`). Direct calls require `dateChrono.create({ config: { fractionalDays: false } })`. With `false`, two times which have the same UTC date have a difference of zero, and 23:00 to 01:00 the next day gives one day. With `true`, that two-hour gap gives `2 / 24` days. `add(from, 0.5)` always adds twelve hours. Use `compare` for precise ordering.

## How normalize participates in scheduling

Normalization maps card or revlog fields to `{ previous, current }`. For Date and Temporal presets:

- During review, `normalize` returns `previous: card.lastReviewAt` and `current: card.dueAt`.
- Elapsed days are `difference(previous ?? current, now)`; New cards always use zero.
- The next card stores `dueAt: add(now, interval)` and `lastReviewAt: now`.
- The revlog stores the pre-review card’s `dueAt` and `lastReviewAt`, plus this review’s `reviewTime: now`.
- During rollback, `normalize` returns `previous: revlog.lastReviewAt` and `current: revlog.dueAt`. These restore the pre-review card’s timestamps; a New card restores `lastReviewAt: null`.

## Try the preset

```ts twoslash file="../../../snippets/run-code/chrono-date.ts"
import { dateChrono } from 'ts-fsrs'

const clock = dateChrono.create({ config: { fractionalDays: false } })
const from = new Date('2026-01-01T23:00:00.000Z')
const to = new Date('2026-01-02T01:00:00.000Z')

console.log(
  JSON.stringify({
    elapsedDays: clock.difference(from, to),
    halfDayLater: clock.add(from, 0.5).toISOString(),
    order: clock.compare?.(from, to),
  })
)

```


▶ Run code

Across midnight, the UTC date difference is one day; adding half a day advances the time by twelve hours.