Scheduling (process order)

Hi experts,

This is the condition of my process order:

Basic start date : 26.05.2005

Basic start finish : 26.05.2005

Scheduled start : 26.05.2005

Scheduled end: 30.05.2015

release : 22.05.2005

i created my process order in 26.05.2005.

process order quantity only 60 roll, it should be taken only a few hours.

But why my Basic start date : 26.05.2005 ?

*it only happen in May 2015, (this months June, it's normal again).

Please help.

Thanks

Yohanna