stock moves seem to be "real time" - no chance to enter a past date.
this is great if everything is entered "real time"
But there are very usual reasons why this might not be the case
* computer/network is down
* inventory - usually at the end of month it takes some hours of the first day of next month to sort everything out.
* your partner (carrier) sends documents for your stock locations at his site a few days later
IMHO OpenERP needs a possibility to enter data for the last period, as long it is not closed.