Daylight Savings time, and how dates are stored internally and displayed
This is probably a question that appears here annually, but I couldn't really find clear answers, so I'll try asking this in my own words:
I'm in the Eastern timezone, and this Sunday, we'll be turning our clocks back an hour at 2:00 AM. That means that accordign to us humans, the time 1:30 AM will occur twice on Sunday.
I've got an Oracle application that runs every 5 minutes around the clock, and it selects records from a certain table whose updated timestamp (TIMESTAMP(6)) is greater than SYSDATE - 5/1440, meaning any record that was updated in the last 5 minutes. Will we have a problem with some records being processed twice on Sunday morning? I'm theorizing that everything will be OK, that internally, Oracle stores DATE fields using something like an epoch which then gets interpreted when we display them. An epoch value will continue to grow each second no matter what “time” it is according to the U.S. Congress.
A simpler way to look at the question might be as follows:
If you store SYSDATE in a DATE column in row “X” at 1:30 AM before the time change, and you store sysdate in row “Y” exactly one hour later, will Oracle say that X’s timestamp is 60 minutes less than Y’s timestamp? All fields that are related to my particular situation are either DATE or TIMESTAMP(6).
We use 11g.