Hi Jane,
I'm afraid with only using to_timestamp it will not be be possible to perform this conversion, because Oracle doesn't supply a direct Date format for this:
DateTime Format Models
However with some extra calculation it is possible to do this. If you know that a numeric value of 1 represents a day in an Oracle date (or timestamp) field you simply can get the Date/Timestamp for your offset (e.g. for 1.1.1970: select to_date('01-JAN-1970','DD-MON-YYYY') from dual and add your milliseconds:
for timestamps you can use the function NUMTODSINTERVAL():
John