Represents the duration of an annotation.
Duration amounts are expressed as an addition of subamounts in different measures. This measures are "year", "day", "hour", "minute", "second". A duration amount expression consists of one or more expressions of the type "*amount* *measure*", where *amount* is a number and *measure* is one of the measures provided before. The total duration is the sum of the subexpressions.
For example, a duration expressed by "(duration "1 day 3 hour 2 second")" represents a duration of 1 day + 3 hours + 2 seconds.
Duration properties belong to the following property types: "DurationProperty", "TimeRelatedProperty".