Hi Patricia,
The planning time fence is there to stop MRP from making changes to requirements that fall within the planning time fence.
Your planning time fence is a rolling period which shifts by 1 day each day and is calculated as of the current date. Yesterdays date is passed so it would not make sense for a planning time fence to use a date in the past to start it's calculation.
Everything that falls before the end of the planning time fence will not be changed by MRP and you will see that all these schedule lines in MD04 will have an * assigned meaning they are firmed for MRP.
If you manually add schedule lines in the past, the planning time fence will not stop you from doing this. The planning time fence will only stop MRP from making changes.
By the way:
If you manually add requirements to the scheduling agreement then MRP will never change these requirements as these are fixed requirements.
If your customer would add requirements in the past or you add independent requirements in the past and your current stock does not cover the requirements the system will generate schedule lines and will put these directly after the planning time fence with an exception message assigned to them.
As a side note:
I always wonder why such questions arrive in the forum as this is something that can be very easily tested in the test environment.