Entering Dates in Sub-Operations 
Use
You can specify in a sub-operation when you want it to start or end in relation to the relevant operation.
To do this, use the start/end reference date together with the start/end offset. The system uses these specifications to calculate the start or end date for the sub-operation.The time references have no influence on the duration of a sub-operation. The actual duration of a sub-operation is specified in the detail data of the sub-operation. It is not used for scheduling an operation. Only the duration specified in the operation is used for scheduling.
Procedure
You are now on the Sub-Operation - Usage dates screen.
Note that you must enter a positive offset for the start of a sub-operation (start of the operation plus n units of time) and a negative offset for the end of the sub-operation (end of the operation minus n units of time).