The following formula works to convert an integer (First_Order_Fcst_Period__c) to a date.  Since we are dealing with fiscal years, with October being the first month of the fiscal year, you will see the calcuation below. 
DATE( 
(If( First_Order_Fcst_Period__c > 9, 
Fiscal_Year_As_Integer__c -1, 
Fiscal_Year_As_Integer__c )) , 
First_Order_Fcst_Period__c ,15)

However, if However, if First_Order_Fcst_Period__c is blank, I am getting is blank, I am getting an error.  I changed my formula to be 
IF(ISBLANK(First_Order_Fcst_Period__c),NULL,
DATE( 
(If( First_Order_Fcst_Period__c > 9, 
Fiscal_Year_As_Integer__c -1, 
Fiscal_Year_As_Integer__c )) , 
First_Order_Fcst_Period__c ,15))

and now I get a compile error.  I suspect it's because First_Order_Fcst_Period__c is rather large

( 
IF(October_Order_Fcst__c > 0 || October_Order_Fcst__c <0,10, 
IF(November_Order_Fcst__c >0 || November_Order_Fcst__c <0,11, 
IF(December_Order_Fcst__c >0 || December_Order_Fcst__c <0,12, 
IF(January_Order_Fcst__c >0 || January_Order_Fcst__c <0,1, 
IF(February_Order_Fcst__c >0 || February_Order_Fcst__c <0,2, 
IF(March_Order_Fcst__c >0 || March_Order_Fcst__c <0,3, 
IF(April_Order_Fcst__c >0 || April_Order_Fcst__c <0,4, 
IF(May_Order_Fcst__c >0 || May_Order_Fcst__c <0,5, 
IF(June_Order_Fcst__c >0 || June_Order_Fcst__c <0,6, 
IF(July_Order_Fcst__c >0 || July_Order_Fcst__c <0,7, 
IF(August_Order_Fcst__c >0 || August_Order_Fcst__c <0,8, 
IF(September_Order_Fcst__c>0|| September_Order_Fcst__c <0,9,Null)))))))))))))

Where is the best way to update this so as not to get a compile error