The SUBDUR operation can be used to subtract a duration specified in factor 2 from a field or constant specified in factor 1 and place the resulting Date, Time or Timestamp in the field specified in the result field.
Factor 1 is optional and may contain a Date, Time or Timestamp field, array, array element, literal or constant. If factor 1 contains a field name, array or array element then its data type must be the same type as the field specified in the result field. If factor 1 is not specified then the duration is subtracted from the field specified in the result field.
Factor 2 is required and contains two subfactors. The first is a numeric field, array or constant with zero decimal positions. If the field is negative then the duration is added to the field. The second subfactor must be a valid duration code indicating the type of duration. The duration code must be consistent with the result field data type. For example, you can subtract a year, month or day duration but not a minute duration from a date field. For list of duration codes and their short forms see Date Operations.
The result field must be a date, time or timestamp data type field, array or array element. If factor 1 is blank, the duration is subtracted from the value in the result field. If the result field is an array, the value in factor 2 is subtracted from each element in the array. If the result field is a time field, the result will always be a valid Time. For example, subtracting 59 minutes from 00:58:59 would give -00:00:01. Since this time is not valid, the compiler adjusts it to 23:59:59.
When subtracting a duration in months from a date, the general rule is that the month portion is decreased by the number of months in the duration, and the day portion is unchanged. The exception to this is when the resulting day portion would exceed the actual number of days in the resulting month. In this case, the resulting day portion is adjusted to the actual month end date. The following examples (which assume a *YMD format) illustrate this point.
The resulting month portion has been decreased by 1; the day portion is unchanged.
The resulting month portion has been decreased by 1; the resulting day portion has been adjusted because April has only 30 days.
Similar results occur when subtracting a year duration. For example, subtracting one year from '92/02/29' results in '91/02/28', an adjusted value since the resulting year is not a leap year.