Returns the number of days from @date1 to @date2 following a 360-day calendar in which all months are assumed to have 30 days.
If method is true, the European method will be used. In this case, if the day of the month is 31 it will be considered as 30.
If method is false or omitted, the US method will be used. This is a somewhat complicated industry standard method.
Note that Gnumeric will perform regular string to serial number conversion for you, so you can enter a date as a string.