Definitions of date arithmetic

[Joe Celko]

>> What is 1 month before 31 March? What is 1 month before 16 March? What
is 1 month before 1 March? What is 1 month after 1 February? What is 1
month after 16 February? What is 1 month after 28 February? <<

The ANSI X3H2 Database Standards committee looked at this and found that
GAAP (Generally Accepted Accounting Practices) and other commercial users
of date arithemetic all disagreed on how to handle months in expression. 
For example, there are over 150 fiscal years, each with slightly different
rules for this.  The Bond trade says a month is always 30 days, etc.  etc.

Our solution in the SQL-92 standard was to only define math with interval
units of one day or less.  Anything more than that was up to the user.

--CELKO--