Requirements date for VSF -- why not on first of month?

Question

Please help me understand this peculiar system behavior. 
MD04 routinely displays the plant/materialu2019s planned independent requirement as a u201CVSFu201D having a requirements date of the first work-day of the period.
What I cannot explain is why the October 2012 VSF has a requirements date of September 21, 2012.  Why would the date not be October 1, 2012?  The factory calendar assigned to the plant has October 1, 2012 as a work-day.
Same situation for December 2012.  The VSF for December has a requirements date of November 21, 2012.
For the rest of 2012, and for the first three months of 2013, the VSF elements are dated on the first work-day of the month.  What would cause the requirements to fall a week and a half early for October and December 2012?
By the way, I am not very fluent in MRP, so please do not assume that I have ruled out something obvious. 
Regards,
zKen

Answer 1

Well, we figured it out.  The problem was the fiscal year variant. 
A few years ago, we were a nine-to-five, Monday-thru Friday kind of company.  As such, the accounting periods were carefully managed so that a period never ended over a weekend or on a holiday.  Nowadays, we are a 24-7 operation, and accounting periods correspond to calendar months.
But when we changed over the fiscal year variant, it was only set up thru 2011.  2012 still had the old period strategy. 
On the MRP3 tab of the material master record, material/plants are assigned the period indicator u201CPu201D, Period according to fiscal year variant.  We forecast by periods in S608. 
MRP was just doing its thing and determining requirements according to our accounting periods and setting the requirements date accordingly.
Solution:  correct the fiscal year variant out for a few more years.  Transport, reload the forecast data, transfer to demand management, and run MRP. 
All is well in the stock/requirements list.
Edited by: zKen on Nov 22, 2011 3:27 PM