Manipulating DATETIME with INTERVAL Values

INTERVAL values can be added to or subtracted from DATETIME values. In either case, the result is a DATETIME value. If you are adding an INTERVAL value to a DATETIME value, the order of values is unimportant; however, if you are subtracting, the DATETIME value must come first. Adding or subtracting a positive INTERVAL value moves the DATETIME result forward or backward in time. The expression shown in the following example moves the date ahead by three years and five months: 
DATETIME (2000-8-1) YEAR TO DAY
   + INTERVAL (3-5) YEAR TO MONTH

Result: DATETIME (2004-01-01) YEAR TO DAY
Important: Evaluate the logic of your addition or subtraction. Remember that months can have 28, 29, 30, or 31 days and that years can have 365 or 366 days.

In most situations, the database server automatically adjusts the calculation when the operands do not have the same precision. In certain contexts, however, you must explicitly adjust the precision of one value to perform the calculation. If the INTERVAL value you are adding or subtracting has fields that are not included in the DATETIME value, you must use the EXTEND function to increase the precision of the DATETIME value. (For more information about the EXTEND function, see the Expression segment in the Informix® Guide to SQL: Syntax.)

For example, you cannot subtract an INTERVAL MINUTE TO MINUTE value from the DATETIME value in the previous example that has a YEAR TO DAY field qualifier. You can, however, use the EXTEND function to perform this calculation, as the following example shows: 
EXTEND (DATETIME (2008-8-1) YEAR TO DAY, YEAR TO MINUTE)
   - INTERVAL (720) MINUTE(3) TO MINUTE

Result: DATETIME (2008-07-31 12:00) YEAR TO MINUTE

The EXTEND function allows you to explicitly increase the DATETIME precision from YEAR TO DAY to YEAR TO MINUTE. This allows the database server to perform the calculation, with the resulting extended precision of YEAR TO MINUTE.