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A date is the representation of a time period, usually of 24 hour duration. This can be absolute (eg 30 December 1899 CE, which happens to be what Google spreadsheets interprets for Day
0) or relative, such as "yesterday" or "tomorrow".
Calculations involving dates are usually based on date serial numbers, with different systems indexing dates from different reference points (UNIX time, the 1900 system, the 1904 system, Hijrah etc) but in each case one unit represents one day and fractions thereof are usually allowed. Hence a quarter of a day (0.25 units) would be 6 hours so, for Google Spreadsheets for example,
1.25 might be formatted to display