DurationConstraint
A DurationConstraint 
defines a ValueSpecification that denotes a duration in time between a start and endpoint. A duration is often an expression representing the number of clock ticks, which may elapse during this duration.
To insert an DurationConstraint:
1.Click the DurationConstraint icon, then click the specific position on the lifeline where the constraint is to be displayed. The default minimum and maximum values, "d..t", are automatically supplied. These values can be edited by double clicking the time constraint, or by editing the values in the Properties window.
2.Use the handles to resize the object if necessary.
To change the orientation of the DurationConstraint:
•Click the "Flip" icon to orient the constraint vertically.
