Oracle: subtract millisecond from a datetime

For adding or subtracting an amount of time expressed as a literal you can use INTERVAL.

SELECT TO_TIMESTAMP('10/08/2012','DD/MM/YYYY')     - INTERVAL '0.001' SECOND FROM dual;

As well there are now standard ways to express date and time literals and avoid the use of various database specific conversion functions.

SELECT TIMESTAMP '2012-10-08 00:00:00'    - INTERVAL '0.001' SECOND DATAFROM dual;

For your original question the time part of a day is stored in fractional days. So one second is:

1 / (hours in day * minutes in hour * seconds in a minute)

Divide by 1000 to get milliseconds.

1 / (24 * 60 * 60 * 1000)

SELECT TO_TIMESTAMP('10/08/2012','DD/MM/YYYY') - NUMTODSINTERVAL(1/(24*50*60*1000),'HOUR') data FROM dual;

OUTPUT

DATA                             ---------------------------------09/AUG/12 11:59:59.999950000 PM  1 row selected.

The answer posted above subtracts a tenth of a millisecond from the date. I think what you want is the following:

SELECT TO_TIMESTAMP('10/08/2012','DD/MM/YYYY')-NUMTODSINTERVAL(1/1000,'SECOND')  FROM dual;

Output:

DATA---------------------------------------------------------------------------09-AUG-12 11.59.59.999000000 PM                   ^^^                   |||              tenths|thousandths                    |                hundredths

The following NUMTODSINTERVAL(1/(24*25*60*1000),'HOUR') seems to work only because 24*25 = 600. But that number is wrong because 1/(600*60*1000) of an hour is a tenth of a millisecond, not a millisecond. If you want to use 'HOUR' in NUMTODSINTERVAL() you should use 1/(60*60*1000) (sixty minutes in an hour, sixty seconds in a minute, 1000 ms in a second).