Question

How does Oracle DB round milliseconds in its date function?

For example if sysdate is run at exactly “05/25/2012 01:15:25.900”, would the date be stored as “05/25/2012 01:15:26” (rounding up) or as “05/25/2012 01:15:25” (rounding down)?

Does it round up for everything beyond half a millisecond, and down for everything lower than half a millisecond?

Here is a simple test I put together:

SELECT to_char(systimestamp, 'dd-mm-yyyy hh:mi:ss:ff') as sys_time_stamp,
to_char(
     TO_DATE (
         TO_CHAR (SYSTIMESTAMP, 'YYYY-MON-DD HH24:MI:SS'),
         'YYYY-MON-DD HH24:MI:SS'), 
     'dd-mm-yyy hh:mi:ss') as sys_date
FROM DUAL;

Run 1
sys_time_stamp: 25-05-2012 12:37:32:798000
sys_date: 25-05-012 12:37:32

Run 2
sys_time_stamp: 25-05-2012 12:43:41:322000
sys_date: 25-05-012 12:43:41

This would indicate that at least when converting a systimestamp to a date the milliseconds are just dropped. But when sysdate is actually generated, are milliseconds actually just dropped?

Answer 1

Rounding the fractions of time up would result in the sysdate being on average half a second fast.

It would flip to the next day at 23:59:59.500.

This would obviously be idiotic, so I’m going to go with truncation rather than rounding.