Obtains a Duration representing an amount in the specified unit.

The parameters represent the two parts of a phrase like '6 Hours'. For example:

  Duration.of(3, SECONDS);
  Duration.of(465, HOURS);

Only a subset of units are accepted by this method. The unit must either have an exact duration or be ChronoUnit.DAYS which is treated as 24 hours. Other units throw an exception.

Parameters:

amount the amount of the duration, measured in terms of the unit, positive or negative

unit the unit that the duration is measured in, must have an exact duration, not null

Returns: a Duration, not null

Exceptions:

DateTimeException if the period unit has an estimated duration

ArithmeticException if a numeric overflow occurs

`amount`		the amount of the duration, measured in terms of the unit, positive or negative
`unit`		the unit that the duration is measured in, must have an exact duration, not null

`DateTimeException`		if the period unit has an estimated duration
`ArithmeticException`		if a numeric overflow occurs