last_day vs '31-JAN-93'??

If I wrote a query with a where clause:

                                                                                        where transdate between '01-JAN-93 and '31-JAN-93'

why would my results differ from:

                                                                                        where transdate between '01-JAN-93 and last_day(sysdate)

Any hints on how the last_day function returns the time component might be helpful.

-Devan

                             
               Devan F. Dewey | Senior Systems Analyst
          CenterLine Software | dewey_at_centerline.com
            10 Fawcett Street | "Leme esplain - no dere is
          Cambridge, MA 02138 | too much. Leme sum up."
                                       -Inigo Montoya