Period closing affects all logistic modules where valuated goods movements are going to take place.
Accounting valuation of material movements is effected on company code level, where the current posting
period is defined, and has to be updated at each beginning of a new period.
Hence, a posting to a prior period is usually not allowed.
However, posting to a prior period is explicitly allowed in IDES systems. You can always post goods movements to the current and previous period.
To identify the current posting period of your company codes please use transaction code OMSY.