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The Excel DAYS360 Function
Related Functions:
DAYSNETWORKDAYSBasic Description
The Excel Days360 function returns the number of days between 2 dates, based on a 360day year (12 x 30 months).
The syntax of the function is:
DAYS360( start_date, end_date, [method] )
where the arguments are as follows:
start_date    The start of the period. 
end_date    The end of the period. 
[method]    An optional logical argument, which gives the method to be used in the calculation. This can be either: FALSE (or omitted)    US (NASD) method used  TRUE    European method used 

US vs. European Method
US Method:
 If the start date is last day of a month it is set to the 30th of that month;
 If the end date is last day of month, then:
 If the start date is the last day of month, the end date is set to the 1st of the following month;
 Otherwise, the end date is set to the 30th of that month.
European Method:
 If the start date is the last day of the month it is set to the 30th of that month;
 If the end date is the last day of the month, then it is set to the 30th of that month.
The financial day count basis rules are explained in detail on the
Wikipedia Day Count Convention pageDate Arguments
Note that the start_date and end_date arguments should be input to the Days360 function as either:
 References to cells containing dates
or Dates returned from formulas.
This is because there is a risk that date arguments that are supplied to Excel functions as text representations of dates may be misinterpreted, depending on the date systems and date interpretation settings on your computer.
Excel Days360 Function Examples
The spreadsheet below shows four simple examples of the Excel Days360 function.
Formulas:  A  B 

1  01Jan2015  =DAYS360( A1, A2 ) 

2  31Jan2015  =DAYS360( A1, A2, TRUE ) 

3  01Feb2015  =DAYS360( A1, A3, FALSE ) 

4   =DAYS360( A3, DATE( 2015, 2, 2 ) ) 

 Results:  A  B 

1  01Jan2015  30 

2  31Jan2015  29 

3  01Feb2015  30 

4   1 


In the above examples:
 In cells B1 and B4 the [method] argument has been omitted. Therefore, the US (NASD) method is used.
 In cell B2, the [method] argument is set to TRUE, so the European method is used. Note that this gives a different result from the function in cell B1, which uses the US (NASD) method.
Note also that, as recommended by Microsoft, in all four calls to the Days360 function, the start_date and end_date arguments have been supplied as either cell references or as values returned from functions (in cell B4, the Date function is used to supply the end_date argument).
Further information and examples of the Excel DAYS360 function are provided on the Microsoft Office website.
DAYS360 Function Errors
If you get an error from the Excel Days360 function, this is likely to be one of the following:
Common Errors
#NUM!    Occurs if either of the supplied start_date or end_date arguments are numeric values, but are not recognised as valid dates. 
#VALUE!    Occurs if either:  One or both of the supplied start_date or end_date arguments are text values that can not be interpreted as dates.
 The supplied [method] argument is nonnumeric.
