How to Substract Milliseconds from A timestamp(3) Field

In PLPGSQL, does anyone know how to subtract MILLISECONDS from a field
defined as timestamp(3), yielding another field defined as timestamp(3)?

Have been trying interval, and conversion functions but no luck.

Example: '2006-10-17 14:23:28.951'; subtract 5 milliseconds from a field
   called mytable.end_time, and return the results to a field named
   work_timestamp.

work_timestamp := ????