Numbers, dates and time

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

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 and Unit systems. The typographic mechanics are spacing, dashes, leading zeros and digit grouping, and they are on Grammar and mechanics. This page is the third piece: how a moment in time is worded.

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

• 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 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.
• 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

• 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

• 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 exists to prevent.
• Never call a range "normal".

Approved / Rejected

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.

Banned words

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.
• 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.
• 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.