---
title: "Numbers, dates and time"
description: "The prose style guide for dates, times, durations and ranges. It also covers the moment when \"2 hours ago\" becomes dangerous."
url: "https://opsinjs.pensievelabs.org/content/numbers-dates-and-time"
source: "https://opsinjs.pensievelabs.org/content/numbers-dates-and-time.md"
section: "Content & language"
kind: "content"
reviewed: "2026-09-20"
reviewer: "content"
aliases: ["date format", "time format", "relative time", "ordinals"]
---
> Elements written as `` below are opsinjs documentation
> components. Their attributes are the content: the values they render are
> generated from `tokens/*.json` and `registry/catalogue.ts` and are
> published separately at https://opsinjs.pensievelabs.org/r/index.json and under the Reference
> section.
> Nothing is missing from this page. The data simply does not live in
> the prose.
## The rule [#the-rule]
**Relative time is a convenience; absolute time is the record. Never let the
convenience replace the record for anything a clinician might read.**
That is the one rule on this page that has consequences beyond tidiness.
Everything else here is style. This one is a correctness rule, and it is why
`RelativeTime` is a component with a specification rather than a formatting
helper.
### The boundary with the clinical rules [#the-boundary-with-the-clinical-rules]
This page owns the **prose style** for dates, times, durations and how a range is
worded. It does not own numeric precision.
How many significant figures a measurement may carry, when a rounded figure
becomes misleading, and how unit conversions are handled are clinical questions
owned by
[Numbers, units and precision](../health/numbers-units-precision.mdx) and
[Unit systems](../health/unit-systems.mdx). The typographic mechanics are
spacing, dashes, leading zeros and digit grouping, and they are on
[Grammar and mechanics](./grammar-and-mechanics.mdx). This page is the third
piece: how a moment in time is worded.
### Dates [#dates]
* **Day, month, year, with the month as a word**: 14 March 2026. Never
14/03/2026 and never 03/14/2026. An all-numeric date is ambiguous between
conventions, and a health record is exactly where that ambiguity is expensive.
* **Drop the year for a date inside the current year**, in an interface where the
reader can see the context. Keep it in anything exported, printed, shared or
older than a few months.
* **No ordinal suffixes.** "1 March", not "1st March".
* **A weekday helps for a near date and hurts for a far one.** "Friday 14 March"
is useful this week; a month out, the weekday is noise.
### Times [#times]
* **24-hour clock with a colon**: 08:12, 17:45. It is unambiguous, sorts, and
matches how appointment letters and device logs are written.
* **A time with no date is only allowed when today is unmistakable.** "Taken at
08:12" on a screen that also says "today" is fine; the same string in a
notification is not.
* **Say the time zone only when it can differ from the reader's.** For a
measurement taken by the reader's own device, it cannot.
### Relative time [#relative-time]
Relative phrasing is easier to read and loses information. The rules:
* **Use relative for recency, within a day.** "2 hours ago", "just now",
"yesterday".
* **Switch to absolute beyond about a day.** "on Tuesday", then "on 14 March".
A relative phrase is unusable once it exceeds the reader's working memory for
counting backwards, which happens quickly.
* **Always pair relative with absolute where the value matters.** "2 hours ago
(08:12)". This costs a few characters and removes the whole class of error.
* **Never use relative alone in anything durable.** An export, a printout, a
share to a clinician and a notification that may be read hours later are all
durable. A printed page saying "2 hours ago" is a page with no time on it at
all. See [Print and export](../accessibility/print-and-export.mdx).
* **Relative time must update or be honest that it does not.** A string rendered
server-side hours before the reader sees it is a lie with a timestamp's
authority.
### Durations and frequencies [#durations-and-frequencies]
* **Words for short durations, numerals with units for longer ones**: "a few
minutes", "20 minutes", "3 days", "6 months".
* **Say the window, not the count, for a trend**: "over the last 7 days", not
"in the last week". The reader needs to know what was included.
* **Frequency as "n times a day", not "n/day"** in prose.
### Ranges [#ranges]
* **A range always uses the word "to"**: "90 to 120 mmHg" in prose, "90 to 120"
in a compact display. The compact form loses the unit, never the relation.
* **Name whose range it is.** "Your usual range", "the range your GP set", "the
range on your results letter". An unattributed range implies a universal
standard, which is exactly the implication
[Reference ranges](../health/reference-ranges.mdx) exists to prevent.
* **Never call a range "normal".**
## Approved / Rejected [#approved--rejected]
Taken 2 hours ago (today at 08:12)
Taken 2 hours ago
14 March 2026
14/03/2026
Your last three readings, on 12, 13 and 14 March
Your last three readings (2d ago, 1d ago, 4h ago)
Your usual range is 90 to 120 mmHg.
Normal range: 90-120.
Over the last 7 days you logged 5 readings.
Weekly adherence: 71%.
## Patterns [#patterns]
**A measurement's timestamp, on screen**
> `relative phrase` (`absolute time`)
**A measurement's timestamp, exported or printed**
> `weekday`, `day` `month` `year` at `HH:MM`
**A range with an owner**
> `Owner's` range is `low` to `high` `unit`.
**A trend window**
> Over the last `n` `days / weeks`, `what happened`.
**Not enough data for a window**
> We need `n` more readings before we can show a trend for `window`.
**A scheduled event**
> `Weekday` `day` `month` at `HH:MM`
**A gap in the record**
> No reading between `date` and `date`.
Say the gap. A chart that joins two points across a two-week hole is asserting
something about the fortnight in between. See
[Uncertainty, staleness and missing data](../health/uncertainty-and-staleness.mdx).
## Banned words [#banned-words]
| Banned | Why | Use instead |
| --------------------------------- | ------------------------------------------------------------------------------- | --------------------------------------- |
| an all-numeric date (14/03/2026) | Ambiguous between conventions in exactly the document where ambiguity costs. | 14 March 2026 |
| 1st, 2nd, 3rd in a date | Noise, and a screen reader reads the suffix. | 1 March |
| am / pm | Two more characters of ambiguity than a 24-hour clock. | 08:12, 17:45 |
| 2d ago, 4h ago, 3w | Telegraphic shorthand in a health record. | 2 days ago, or the date |
| recently, a while ago, lately | Unquantified time in a record of measurements. | the window, or the date |
| a bare relative time in an export | A printed page with no time on it. | absolute, or both |
| normal range | A verdict, and unowned. See [Reference ranges](../health/reference-ranges.mdx). | your usual range, the range your GP set |
| week (for a 7-day window) | Implies calendar weeks, which is usually not what was computed. | the last 7 days |
## Related components [#related-components]
* **`RelativeTime`** takes every rule in the Relative time section above as a
requirement, including the hardest one: knowing when to refuse to be relative.
* **`Value`** covers the number and its unit; the typography is on
[Grammar and mechanics](./grammar-and-mechanics.mdx).
* **`TrendSparkline`** takes the window wording and the gap wording as its
content contract, and the minimum number of readings before a trend may be
drawn is a clinical rule from
[Trends and change](../health/trends-and-change.mdx).
* **`LogSheet`** and **`ReadingInput`** capture a moment; the date and time
format used at capture is the one that ends up in every export.
* **`RangeBar`** displays a range, and must name whose range it is.