The Document Date
The Document date is the date which is appearing on the invoices. The document date is used to set the default due date based on terms and used for calculating the discounted amount and also used for aging purposes. By using document date we can also print posting journals.
New Stuff: Bulk PO Receipts feature for Sage 300 ERP
The Posting Date
The Posting Date is the date which is used to post the transaction in the General Ledger. The Posting date also determines the fiscal year and period to which the transaction is posted.
Accounts Payable selects open documents for revaluation using posting dates instead of document dates, so that exchange adjustments entries will be posted to the correct fiscal year and period.
Depending on the selection of the Default Posting Date option on the A / P Options screen, the default posting date may be the date of the document, the batch date or the date of the session. 
The most common reason for a different post date than a document date is that the fiscal year in which the document date falls has already been locked.
But, since the invoice must be paid in terms or may be eligible for a discount, it is important not to change the date of the document for an open fiscal year. One of the common reasons for having the posting date different from a Document Date is, when we receive an invoice for the month of February 28 2019 but we are entering it on March 1st 2019.
If we put the Posting date as February 28, 2019 than company expenses will increase on February month. If we put the posting date as March 1st, 2019 company expense will increase on March month accounts.
In this way we can differentiate posting date and document data.
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