planned order basic dates and production dates

[tn12]

Hi Experts,

We are trying to figure out the issue happening when converting planned order to production order.
Please help us with the mechanisms of calculating the basic dates between planned order and production order.

<Situation> 
There are two planned order which basic dates are as follows:
They are same material, routing, BOM, issue quantity, float(which is 0 days), production lead time (8days)
1:Planned order A
Basic start date:9/6
Basic end date:9/9

2:Planned order B
Basic start date:9/3
Basic end date:9/6

When we convert them to production order, the basic dates are adjusted regarding the production lead time with backwards:
1:production order A (correct calculation)
Basic start date:8/29 (8 workdays from 9/9)
Basic end date:9/9 (Same end date as planned order)

2:Production order B (weird calculation)
Basic start date:8/27 (9 workdays from 9/6)
Basic end date:9/5 (1 day earlier than planned order)

Does anyone know why production order B have calculated 9days from the basic end dates of planned order?

we have checked following attributes;
>customized calendar, and no holidays are set during these days.
>Float are all 0 days

Thanks in advance.

[itzemuetze2]

hi tn12, Please Check 

https://me.sap.com/notes/46237/E

For further Details on how Dates are determined.

hope this Helps