RE: AP Purchasing Invoice Entry
This is often dictated by a company's accounting practices, so you should always follow what your company specific guidelines are.
That said, typically, the document date is entered as the real invoice date (Feb 1 in your example) and this is what is used to post the transaction to the AP subledger - as you say, otherwise the AP aging will be incorrect. The GL Posting Date (also called Posting Date on many windows in GP) can be completely different from the Document Date and the GL Posting Date is what determines when this transaction will hit the General Ledger. In your example, this would be March 1 if February is already closed.
It could also be a different date based on when the expense is incurred. For example - on March 17 we get an invoice for health insurance covering April 1 through April 30. We enter the invoice with a Document Date of March 17, but with a GL Posting Date of April 1 so that on our P&L Statement this shows up as an expense for April. Having the Document Date of March 17 will allow us to pay it prior to the due date of April 1, as the Due Date calculations in GP will use the Document Date.
Where the GL Posting Date is specified for AP transactions can be different depending on the posting settings of your GP company. As Richard says, sometimes it is on the Payables Batch, however sometimes it is on each transaction (that is what we use to allow us to have transactions for multiple GL Posting Dates in the same AP batch).
Hope this helps give you some more information. Again, how you should enter transactions should be dictated by your company's accounting practices. There may be very good reasons why your company uses slightly different rules regarding how dates are entered for AP transactions.