DateTime expressions formed by adding an Interval to a DateTime value or by subtracting an Interval from a DateTime value are performed by adding or subtracting values of the appropriate component fields and carrying overflow from lower precision fields with the appropriate modulo to represent proper arithmetic in terms of the calendar and clock.
An interval_expression or interval_term may only contain DateTime fields that are contained in the corresponding date_time_expression or date_time_term.
When an Interval value is added to or subtracted from a TIME or TIMESTAMP value, the time zone displacement value associated with the result is identical to that associated with the TIME or TIMESTAMP value.