How to subtract seconds from postgres datetime

select '2014-04-14 12:17:55.772'::timestamp - interval '2 seconds';

For greater flexibility it is possible to mutiply the interval

select '2014-04-14 12:17:55.772'::timestamp - 2 * interval '1 second';

If you want to truncate to the second

select date_trunc(
    'second', 
    '2014-04-14 12:17:55.772'::timestamp - interval '2 seconds'
);

Postgres does not have a dateTime data type. I assume you mean a timestamp.

You can subtract an "interval" with the desired length from that column:

select the_timestamp_column - interval '2' second
from the_table

More about intervals in the manual

More about the operators available for date and timestamp columns in the manual