Keep the following rules in mind.
When you issue date arithmetic statements using durations, do not try to establish a common conversion factor between durations of different types. For example, the following two date arithmetic statements are not equivalent:
1997/04/03 - 1 MONTH
1997/04/03 - 30 DAYS
April has 30 days, so the normal response would be to subtract 30 days to subtract one month. The result of the first statement is 1997/03/03, but the result of the second statement is 1997/03/04. In general, use like durations (for example, use months or use days, but not both) when you issue date arithmetic.
Another consideration: if one operand is a date, the other operand must be a date or a date duration. If one operand is a time, the other operand must be a time or a time duration. You cannot mix durations and data types with date and time arithmetic.
If one operand is a timestamp, the other operand can be a time, a date, a time duration, or a date duration. The second operand cannot be a timestamp. You can mix date and time durations with timestamp data types.
Now, what exactly is in that field returned as the result of a date or time calculation? Simply stated, it is a duration. There are three types of durations: date durations, time durations, and labeled durations.
Date durations are expressed as a DECIMAL(8,0) number. The result of subtracting one DATE value from another is a date duration. To be properly interpreted, the number must have the format yyyymmdd, where yyyy represents the number of years, mm the number of months, and dd the number of days.
Time durations are expressed as a DECIMAL(6,0) number. To be properly interpreted, the number must have the format hhmmss, where hh represents the number of hours, mm the number of minutes, and ss the number of seconds. The result of subtracting one TIME value from another is a time duration.
Labeled durations represent a specific unit of time as expressed by a number followed by one of the seven duration keywords: YEARS, MONTHS, DAYS, HOURS, MINUTES, SECONDS, or MICROSECONDS. A labeled duration can only be used as an operand of an arithmetic operator, and the other operand must have a data type of DATE, TIME, or TIMESTAMP. For example:
CURRENT DATE + 3 YEARS + 6 MONTHS
This will add three and a half years to the current date.