DAYS360 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.
This function is Excel compatible.