The Central European Daylight Saving Time begins on last Sunday in March. We set our clocks from 02:00 to 03:00. What happens if I do timestamp calcuations in a database request - lets say at 01:59?
UPDATE sessions SET aliveuntil = (CURRENT_TIMESTAMP + INTERVAL '1' MINUTE) WHERE id = ?
Do I get 03:00 as result or 02:00?
And what about the over way around if we set our clocks from 03:00 to 02:00?
SELECT id FROM sessions WHERE aliveuntil < (CURRENT_TIMESTAMP - INTERVAL '1' MINUTE)
After time changed from 03:00 to 02:00... what happens with the (CURRENT_TIMESTAMP - INTERVAL '1' MINUTE) at 02:00? Is it 02:59 or 01:59?
How should this be handled? Best practice and how it's handle (in my particular case) by Oracle Database 11g Release 11.2.0.2.0?
If I'm understanding their documentation correctly, it depends on how the table/columns are setup in the database. If the columns are setup to use WITH TIME ZONE, then Oracle automatically determines the correct/relevant values. In your above example if the column aliveuntil has this setting, then if you try to add 1 minute at 1:59, the time will update to 3:00.
Here is a helpful article I found on the subject:
http://docs.oracle.com/cd/B19306_01/server.102/b14225/ch4datetime.htm
Scroll down to the bottom of the article and you should see what you're looking for.
Here was a section of the article I found relevant:
For example, in the Eastern region of the United States, the time changes from 01:59:59 a.m. to 3:00:00 a.m. when Daylight Saving Time goes into effect. The interval between 02:00:00 and 02:59:59 a.m. does not exist. Values in that interval are invalid.
来源：https://stackoverflow.com/questions/14157708/timestamp-calculation-with-daylight-saving-time