Costing Release (CK24) error

Dear Experts

I have opened Aug15 using MMPV on 31st July.

After that when costing team try to run costing for a product on 31072015, it was giving error during release of the same (CK24)

ERROR:Company code xxxx : period closing has not yet taken place

I wanted to know one thing if i opened MM Period (aug15-period 5) on 31st July,15, will SAP allow run the costing for July since System date is still July (31072015). if YES, then why i got the above error.

Also would like to know that is there any harm/disadvantage in opening future MM period on previous day of the next month (as this case)

Pl advice

Thanks

Venu