SA Schedule Lines from MD03 date calculation

Question

Hi
Please find the following snap from MD03 ,
here the schedule lines generated dt 06-06-14, for this material planned delivery time and GR processing time is zero in material master,
In info record planned delivery time is 1 day, I unable to understand why system generated the schedule after 4 days
Then I created new material copying the above material and done MD03 for newly created then the schedule lines generated on 02-06-14, Why the system proposes two different schedule dates for new material and ol material?
I searched the forum and checked all the available configurations and finally found
IMG-Production-MRP-Planning-scheduling and capacity parameters-Parameters for determining the basic date
when i checked the box for Start in past and again run the MD03 for the same material, then the results as follows, which is correct i think as per MMR
Though i got the required results but did not understand what is this field Start in Past does? how the calculations takes place, what will be the effect if i keep this checked ?
Thanks

Answer 1

But how these fields were populated?? I think at the time of creation of SA, material master has the data and it has been carried forward, is it right
Either a user manually maintained the values that you have in the SA or these values where at time of creation present in either the material master or the info record.
The system (depending on the config set-up) copies the values in the following sequence to the SA.
If values are maintained in the info record the system will adopt the data from the info record
If no values in the info record then the system will copy the data from the material master record.
I can't judge from here what values where maintained where. You can check the change logs to check if you can find the values back. In case you can not find the values in the change logs then it's likely that the values where manually maintained in the SA.
But How this Parameters for determining the basic date work ?
The determination of the date works based on the backward and forward scheduling.
The system will first schedule using backward scheduling if this is not possible then forward scheduling is used.
Some explanation on backward and forward scheduling.
Backward scheduling explained
When scheduling materials in SAP MRP, the system will first try to backward schedule the materials.
With backward scheduling is meant that the SAP MRP system will start to count backward from the
requirement date and will then determine the delivery date. As long as this date is later then the current
date, the delivery date will be determined without a problem.
Below image shows a graphical representation of backward scheduling.
Forward scheduling explained
Forward scheduling is used by the SAP MRP system when the date determined by the backward
scheduling process is lying before the actual date. The forward scheduled date is determined by taking
today’s date and then adding all the relevant processing time to determine the possible delivery date.
This kind of scheduling will throw an exception message in the MRP view screen (MD04 / MD06 /
MD07). These exception messages are telling the materials expeditor that action from her/his part is
required in order to either postpone the production of the finished good or to work with the vendor to
pull forward the required materials.
Below image shows a graphical representation of how the forward scheduling is performed by SAP.