Date and time notation in the United Kingdom
In the United Kingdom, the date is written using the 'DMY' format, eg 21 October 2010 or if abbreviated, a stroke is used to separate numbers, eg 21/10/10 or 21/10/2010. The time can either be written using 12-hour, eg 4.10pm, or 24-hour notation, eg 16.10. Note, however, that 24-hour time is rarely spoken except within industries that normally use it, such as rail and bus transport, or the military.
Dates are written traditionally in 'day month year' order. This order is used in both the traditional all-numeric date (eg '31/12/99') and the expanded form (eg '31 December 1999'). The Guardian Style Guide recommends you use this format (DMY; no commas). Writing the day of the month as an ordinal number (eg '31st December') is also very common - and since the advent of automatic correction in word processors, the ordinal indicator has been lifted into superscript (eg '31st December') in typed documents, to match the handwritten style. Single digit numbers for day or month may have a preceding nought (eg '09/09/2015'), but omitting it (eg '9/9/2015') is more usual in handwritten documents. You should not write in the 'dd/mm/yy' format (eg '09/09/15') because you are lengthening the day and month, whilst shortening the year.
When saying the date, it is usually pronounced using 'the', then the ordinal number of the day first, then the preposition 'of', then the month (eg 'the 31st of December'). The month-first form (eg 'December the 3rd') was widespread until the mid twentieth-century, and remains the most common format for newspapers across the United Kingdom. The month-first format is still spoken, perhaps more commonly when not including a year in the sentence, but is now less frequently used, having been superseded by the little-endian DMY format in most cases.
The 'day month year' order is also the case in modern Welsh (eg '20 Mai 1999', '20fed Mai 1999', '20fed Mai 1999'). The 'month day year' order (eg 'Mai 20, 1999') was previously more common than it is nowadays, it not being unusual to see a Welsh 'month day year' date next to an English "day month year" date on a bilingual plaque from the latter half of the 20th century.
20 Mai 1999' is read as 'yr ugeinfed o Fai mil naw naw naw' with the usual soft mutation of M to F after 'o' (of). 1999 can be read as either 'mil naw naw nawnaw' (thousand nine nine nine) or 'un naw naw naw' (one nine nine nine).
Weeks are generally referred to by the date on which they start, referring to the Monday, eg 'week commencing 5 March'. ISO 8601 week numbers are found in diaries and are used in business.
ISO 8601 has been adopted as British Standard BS ISO 8601:2004, and is popular in specialist use (eg use-by dates on medical products) and computer applications eg database systems, communication protocols, web pages, and so forth. The appearance of such big-endian dates is increasing especially with computer-generated forms and invoices, as well as simply internet-accessible content in an age of Globalisation.
Both the 12-hour and 24-hour notations are used in the United Kingdom. The 12-hour notation is still widely used in ordinary life, written communication and displays, and continues to be used in spoken language. The 24-hour notation is used in timetables and in some computer applications; computers running Microsoft Windows with UK regional settings default to display time in 24-hour notation. The 24-hour notation is used more often than in the United States - especially for bus, train and airline timetables - but not quite as commonly as in much of the non-English speaking world.
It is rare to use the 24-hour format when speaking; to convey the time 21.30 you convert to 12-hour format ('half past nine') rather than say 'twenty-one thirty'. However, the spoken 24-hour format is used exclusively in airport and railway station announcements: 'We regret to inform that the fifteen hundred (15.00) service from Nottingham is running about 10 minutes late' / 'The next train arriving at Platform four is the twenty fifteen (20.15) service to London Euston'.
To separate different parts of time, either a full stop or a colon can be used, depending on whether it is 12 or 24-hour notation. A style that many people like to adopt is that for 12-hour time a full stop is used (eg 1.45pm), whilst a colon is used for 24-hour time denotation (eg 13:45). Such formats are used by institutions including the BBC and the Evening Standard. 
This is, however, not a definite rule; a full stop may be used for 24-hour time (the Guardian Style Guide recommends a full stop for both 12-hour and 24-hour time). A colon must not be used for 12-hour time, eg 7:30pm would be incorrect.
You must not mix notations: '07.30pm' uses a leading nought (0) and a 'pm' suffix, so combines 12-hour and 24-hour time in a way in which will confuse readers.
In British English, the expression 'half [hour]' is used colloquially to denote 30 minutes past the hour. For example, 'half ten' means '10.30' (without specifying morning or night). This contrasts with the Dutch, German, Hungarian, Czech, Baltic and Scandinavian languages, where the same type of expression denotes 30 minutes before the hour.
The Welsh language usage of the 12-hour and 24-hour clocks is similar to that of UK English above. However, the 24-hour notation is interesting in that it only has a written, not a spoken form. For example, written 09.00 and 21.00 are both said ('naw o'r gloch' o'clock, literally nine of the bell). Minutes are always either 'wedi' (after) or 'i' (to) the hour, eg 21.18 'deunaw (munud) wedi naw' (eighteen (minutes) past nine) and 21.42 'deunaw (munud) i ddeg' (eighteen (minutes) to ten). Phrases such as 'y bore' ((of) the morning), 'y prynhawn' ((of) the afternoon) and 'yr hwyr'((of) the evening) are used to distinguish times in 12-hour notation, much like Latin 'am' and 'pm', which are also in common use, eg 9.00yb (09.00) as opposed to 9.00yh (21.00).
- Marsh, David; Hodsdon, Amelia. Guardian Style. Guardian Books. p. 87. ISBN 9780852652220.
- International Standard ISO 15223: Medical devices – Symbols to be used with medical device labels, labeling and information to be supplied
- "BBC News Style guide". Retrieved 27 June 2017.
- "BBC Skillswise" (PDF).
- Marsh, David; Hodsdon, Amelia. Guardian Style. Guardian Books. p. 297. ISBN 9780852652220.