Converts this instant to the number of milliseconds from the epoch of 1970-01-01T00:00:00Z.

If this instant represents a point on the time-line too far in the future or past to fit in a long milliseconds, then an exception is thrown.

If this instant has greater than millisecond precision, then the conversion will drop any excess precision information as though the amount in nanoseconds was subject to integer division by one million.

Returns:  the number of milliseconds since the epoch of 1970-01-01T00:00:00Z

Exceptions:

ArithmeticException

if numeric overflow occurs