Office Move summarizes all of its detail tasks (Equipment, and Cubicles/Desks), and if (for instance) Equipment were to be given any detail tasks, Equipment would become a summary task that summarizes those detail tasks. The start date of Office Move will automatically be calculated by the software to be the same as the earliest start date of all of its detail tasks the end date will be calculated to be the same as the latest end date of all its detail tasks. Quite simply, a summary task's Duration is just the duration (per the Master Calendar) between these start and end dates. Since some beginners have difficulty grasping this concept, we go into more detail about it (and its implications) in the next couple of paragraphs.
The duration of a summary task is usually not the same as the sum of the durations of its detail tasks. This is because a summary's detail tasks will often NOT happen one right after the other in time, but overlap in time, or have gaps of time between them. For instance, if the first detail task has zero duration, and the last detail task happens to end 10 days later than the first task starts, the duration of the summary is 10 days, no matter what any of the other detail task durations are. However, all costs and effort hours DO sum up (roll up) arithmetically to the summary. For instance, if one detail task has 2 effort hours in it and a second detail task has 3 effort hours in it, 5 hours total from the detail tasks will roll up to the summary, in the Effort Hours column.
Entering a start date into a summary determines the earliest possible date that any ASAP task in the family can occur. By NOT entering a Start or End date into a summary task, the summary simply inherits as its start date the earliest start date of all of its detail tasks, and inherits as its end date the latest end date of all of its detail tasks. Users who enter a start date into a summary task should be aware that the summary's start date can be overridden by a fixed date in any one of its detail tasks. This is potentially confusing because it is possible for a summary's fixed start date to be different from the date that the summary is actually scheduled to start (as defined by its earliest detail task). If this seems too confusing, you can set the summary task to ASAP in the Timing Info Box, or add an ASAP detail task to the family that represents the earliest, real activity in the family. (If the earliest detail task has already started, set its Percent Complete to > 0.)
Note that physically dragging an ASAP or ALAP task bar (whether a detail or summary) will cause it to become Fixed, and have a Start Date. Fortunately, if you drag a summary task that has one or more fixed details, the software will automatically ask you if and how you want to reschedule the fixed details.