Increment and decrement dates

The result of adding a duration to a date, or of subtracting a duration from a date, is itself a date.

(For the purposes of this operation, a month denotes the equivalent of a calendar page. Adding months to a date, then, is like turning the pages of a calendar, starting with the page on which the date appears.) The result must fall between the dates January 1, 0001, and December 31, 9999, inclusive. If a duration of years is added or subtracted, only the year portion of the date is affected. The month is unchanged, as is the day unless the result would be February 29 of a year that is not a leap year. In this case, the day is changed to 28.

Similarly, if a duration of months is added or subtracted, only months and, if necessary, years are affected. The day portion of the date is unchanged unless the result would not be valid (September 31, for example). In this case, the day is set to the last day of the month.

Adding or subtracting a duration of days, of course, affects the day portion of the date, and potentially the month and year.

Date durations, whether positive or negative, can also be added to and subtracted from dates. As with labeled durations, the result is a valid date.

When a positive date duration is added to a date, or a negative date duration is subtracted from a date, the date is incremented by the specified number of years, months, and days, in that order. Thus, DATE1 + X, where X is a positive DECIMAL(8,0) number, is equivalent to the expression: DATE1 + %DURYEAR(%YEAR(X)) + %DURMONTH(%MONTH(X)) + %DURDAY(%DAY(X))

When a positive date duration is subtracted from a date, or a negative date duration is added to a date, the date is decremented by the specified number of days, months, and years, in that order. Thus, DATE1 - X, where X is a positive DECIMAL(8,0) number, is equivalent to the expression: DATE1 - %DURDAY(%DAY(X)) - %DURMONTH(%MONTH(X)) - %DURYEAR(%YEAR(X))

When adding durations to dates, adding one month to a given date gives the same date one month later unless that date does not exist in the later month. In that case, the date is set to that of the last day of the later month. For example, January 28 plus one month gives February 28; and one month added to January 29, 30, or 31 results in either February 28 or, for a leap year, February 29.

Note: If one or more months are added to a given date and then the same number of months is subtracted from the result, the final date is not necessarily the same as the original date.