Scheduling Dates for Planned Order and Production orders

Hi,

I need some clarifications about scheduling.

I maintained the following details for the Product in MM

GR =1 , Inhouse =1, Planned Deliver=1 and Scheduling Margin Key = 1 + 1+ 1 +1.

and In routing i have 2 operations Operation 10 setup time i have given 30mts and for Operation 20 i didint maintain any standard timings.

I have given requirement on 30.01.2009.

When i run the MRP on Basic Scheduling (MD02 - Scheduling indicator is 1)

Planned Order

Ord Finish 29.01.09

Ord Start 28.01.09

Pla Open 27.01.09

Pro Order (Basic Date)

Basic St 28.01.09

Basic Fi 29.01.09

Production Order Sch

28.01.09 14.20Hrs

28.01.09 15.00Hrs

27.01.09 (Release)

When i run the MRP on Lead Time Scheduling

Planned Order

Ord Finish 29.01.09

Ord Start 26.01.09

Pla Open 23.01.09

Pro Order (Basic Date)

Basic St 26.01.09

Basic Fi 29.01.09

Production Order Sch

27.01.09 13.55Hrs

27.01.09 15.00Hrs

26.01.09 (Release)

Can any one explain the dates how the system has calculated.

Thanks in adavance

Nag