[Lizard]

Until we add a feature to split up tasks to represent the interruptions that inevitably seem to occur, you have three options:
(And in OmniPlan, duration is always end date minus start date, so I'm going to talk about how to make the effort correct instead.)

1) Keep the effort accurate and lie about either the start or end date.
2) Keep the start and end dates accurate and ignore the effort field.
3) Set the start and end dates. Then adjust the resource's assigned percentage to the task until effort is also correct.