WIP/ Variance calculation for production orders

Hi Experts,

Would like to have your valued opinion on the below: with respect to Period end closing of Production orders ?

Period-1 /2012 : 01-Jan-2012 to 31-Jan-12.

Period -2 /2012 : 01-Feb -2012 to 29-Feb-12

31-01-12 : Goods issued to process order, Partial goods recieved. ( Period-1)

01-02-2013 : Some more goods issued to process order and Goods recieved in full. Delivered fuly.( Period-02)

07-02-2013 : Run WIP/ Variance calculation program for Period-01/2012

Whether system calculates WIP or Variance ? Just wanted to check whether the status change has any time stamp ? How does SAP system know that it has to calculate WIP on the last date of the period for the prodn order ?

Regards