Map elapsed life to the instant whose positions are progressed: one
365.2422-day tropical year advances one 86,400,000-millisecond day after
birth, both counted in UTC milliseconds as Date counts them (no leap
seconds), not in ephemeris days. This is elapsed-time arithmetic, not
calendar-year or local civil-time arithmetic. Targets before birth use the
same signed mapping, which is not a converse progression. Inputs are
resolved DateInput instants; they and the returned Date are independent objects.
Map elapsed life to the instant whose positions are progressed: one 365.2422-day tropical year advances one 86,400,000-millisecond day after birth, both counted in UTC milliseconds as Date counts them (no leap seconds), not in ephemeris days. This is elapsed-time arithmetic, not calendar-year or local civil-time arithmetic. Targets before birth use the same signed mapping, which is not a converse progression. Inputs are resolved DateInput instants; they and the returned Date are independent objects.