about nvl2 function

sorry , I'm new to oracle world..

who could help explain following.. I expected following SQL would error out.. but I got following result.

 I don't understand, how date format string '1/1/2010' could convert to number .000497512, and how date format string 1-1-2010 could convert to number -2010?

SQL> select nvl2(birth_dt,0,1/1/2010) from eshen.test;

NVL2(BIRTH_DT,0,1/1/2010)

.000497512
.000497512

SQL> select nvl2(birth_dt,0,1-1-2010) from eshen.test;

NVL2(BIRTH_DT,0,1-1-2010)

                    -2010
                    -2010