There are few ways to calculate end of day i.e when the time is 23:59:59. In the example below it will return a value a value of 01-APR-2009 23:59:59.

to_date(’01-APR-2009′) + 1 – 1/86400
to_date(’01-APR-2009′) + 0.99999
to_date(’01-APR-2009 23:59:59′, ‘DD-MON-YYYY HH24:MI:SS’)
trunc(sysdate) + 0.99999 – end of today