Numbers, Dates & Times Quick Rules

Is your birthday 5/3 or 3/5? Is the match at 9.30 or 9:30? Numbers, dates and times have a few small rules — and a couple that flip between British and American English. This is the quick card to check before you write a date on a form or a time in a message. For the marks themselves, the full guide is The Punctuation Map.

Spelling numbers out

• Small numbers, spell them out; large ones, use digits. A common school rule is: words for one to nine, digits for 10 and up. I have three pens and 15 books.
• Never start a sentence with a digit — spell it or reword. Fifteen students came (not 15 students came).
• Hyphenate two-word numbers from twenty-one to ninety-nine: thirty-four, sixty-seven. → Hyphens & Compound Words

Dates — the big UK/US flip

This is the one that causes real mix-ups: 3/5 is March 5th to an American and the 3rd of May to a Brit. When it matters, write the month in words — 5 March is clear to everyone.

Times

• British English often uses a full stop: 9.30am. American English uses a colon: 9:30am. Both are clear; a colon (9:30) is the safe choice everywhere. → End Punctuation
• 24-hour clock (14:30) needs no am/pm. 12-hour clock needs am or pm (2:30pm).
• Say half past nine, write 9:30; quarter to four is 3:45.

Decimals and big numbers

• Both varieties use a full stop for the decimal point (3.5, £4.99) and a comma to group thousands (10,000). That part does not flip.
• Percentages: 25% or 25 per cent (UK) / 25 percent (US).

The takeaway

Only the date order and the time separator really change between British and American English — the rest is the same. When a date could be misread, write the month in words, and keep whatever style you pick consistent across the whole piece.

A few more quick ones

• Ordinals — 1st, 2nd, 3rd, 4th; spell them out at the start of a sentence (First, we…).
• Money — £4.99, $5, 50p; keep the symbol with the number.
• Ages and scores — digits are fine even when small: she is 9, we won 3–0.

Ranges, fractions and lists

• Ranges — use a dash and no spaces: pages 10–12, the 2019–2020 season, aged 5–7.
• Fractions — hyphenate when written out: two-thirds, a half.
• Phone numbers — keep the digits grouped as people expect (07700 900123); do not run them together.
• In a list — be consistent: either spell all the numbers or use digits for all, rather than mixing three and 4 in the same sentence.

Where next

• Punctuation — the young-learner hub — the bigger map this sits inside
• The Punctuation Map — every mark and its job
• Hyphens & Compound Words — hyphens in numbers and compounds
• End Punctuation — full stops in decimals and abbreviations