JMP uses seconds as the measure of time and the basis for dates. The value 0 is equivalent to 12:00 AM January 1, 1904. So the subtract function can take the difference in seconds between two time or date values. Similarily, the add function can be used to directly increment a time or date value by one second.
You can easily enter a literal value (constant) using the DDMmmYYYY:HH:MM:SS.S form, so 25Jun2018 is today.