For most of these functions, you can specify the date argument as a DateTime object, as in the result of a function such as DATE(2003,7,4), or a TimeSpan object, as in the result of a function such as TIME(12,0,0). For compatibility with Excel, it also allows dates to be specified as a number (as in 37806.5) or as a string (as in "7/4/2003 12:00"). The numbers and strings are converted to instances of the DateTime class.
Dates as numeric values are in the form x.y, where x is the "number of days since December 30, 1899" and y is the fraction of days. The numbers to the left represent the date. Times as numeric values are decimal fractions ranging from 0 to 0.99999999, representing the times from 0:00:00 (12:00:00 A.M.) to 23:59:59 (11:59:59 P.M.).
The following three formulas produce the same result:
In Excel, dates can range from 01/01/1900 to 12/31/9999, and in the .NET framework, instances of the DateTime class can range from 01/01/0001 to 12/31/9999. In SpreadJS, we generally support the larger range found in the .NET framework. For Excel compatibility, there are a few cases where the function allows only the smaller range (for example, the DATE function can only be used to enter dates since 01/01/1900).
Both Excel and OLE automation use doubles to represent dates and times, with the integer portion of the double representing the number of days from a base date. In Excel, the base date that is used is 01/01/1900 and the year 1900 is treated as a leap year. In OLE automation, Microsoft corrected this by using the base date of 12/31/1899. As OLE automation does, our spreadsheets treat 1900 as a non-leap year and thus use the base date of 12/31/1899.