Wikipedia on the 24-hour clock

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Time-of-day notations beyond 24:00 (such as 24:01 or 25:59) are neither commonly used nor covered by any relevant standards, although they have been seen on rare occasions in Japan. However, in producing television programmes in Hong Kong, especially for drama series or situation comedies (which often require the crew and the cast to work until very late night), such notations beyond 24:00 are commonly used orally. An example is “I (have got to) work till 28 tonight.” (i.e. 04:00 on the next day)