Date counting conventions in OOXML

[robert_weir@us.ibm.com]

Ecma, as part of the OOXML process in JTC1, has recently
proposed some additional language for their spreadsheet formula description
in the area of date counting conventions for financial functions.  We
should compare this to what we have in OpenFormula and see whether we can
harmonize our definitions.

Note that this is not necessarily the final word from
JTC1, but it is the fullest description we have from them so far.

Does anyone have a good idea what the statement, "If
the date is 28 or 29 February, it is adjusted to 30 February" means?
 Once you break outside of the Gregorian calendar and introduce fictitious
dates like 30 February, you really need to specify their semantics.  For
example, is 30 February an alias for 1 March or 2 March?  Or is it
an new intercalary date interposed between 29 February and 1 March?  

-Rob

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(0) US (NASD) 30/360 -- 

Assumes that each month has 30 days and the total
number of days in the year is 360 by making the following adjustments:

 - If the date is 28 or 29 February, it is adjusted
to 30 February.
 - For months with 31 days, if the first date
has a day value of 31, the date is converted to day 30. If the second date
has a day value of 31, it is changed to 30 days as long as the first date
was not 28 or 29 February, in which case it does not change.

(1) Actual/actual --

The actual number of days between the two dates are
counted. If the date range includes the date 29 February, the year is 366
days; otherwise it is 365 days.


(2) Actual/360 -- 

Similar to Basis 1, but only has 360 days per year.


(3) Actual/365 -- 

Similar to Basis 1, but always has 365 days per year.



(4) European 30/360 -- 

The European method for adjusting day counts. Assumes
that each month has 30 days and the total number of days in the year is
360 by making the following adjustments:

- If the date is 28 or 29 February, it is adjusted
to 30 February.
- For months with 31 days, all dates with a day value
of 31 are changed to day 30, including 28 and 29 February.