To eliminate the ambiguity of boundary cases when the time switches from Standard Time to Daylight Saving Time, use both the TZR format element and the corresponding TZD format element. The TZD format element is an abbreviation of the time zone region with Daylight Saving Time information included. Examples are PST for US/Pacific standard time and PDT for US/Pacific daylight time. The following specification ensures that a Daylight Saving Time value is returned:

TIMESTAMP '1999-10-29 01:30:00 US/Pacific PDT'

If you do not add the TZD format element, and the datetime value is ambiguous, then Oracle returns an error if you have the ERROR_ON_OVERLAP_TIME session parameter set to TRUE. If ERROR_ON_OVERLAP_TIME is set to FALSE (the default value), then Oracle interprets the ambiguous datetime as Standard Time.