Dates and times are stored internally as different parts of a real number.
The value to the left of the decimal represents a date between December 30, 1899 and December 30, 9999, inclusive. Negative values represent dates prior to December 30, 1899.
The value to the right of the decimal represents a time between 0:00:00 and 23:59:59, inclusive. Midday is represented by .5.