If you entered payment terms on the vendor's record, QuickBooks uses the payment terms to determine the due date. For example, if the terms are Net 30 (which means, “balance due in 30 days”) and the date of the bill is January 1st, QuickBooks enters the due date as January 31st.
If you did not specify payment terms for the vendor, QuickBooks calculates the due date by using the “Bills are due __ days after receipt” preference (choose QuickBooks > Preferences and then click Transactions). For example if the preference is set to 10 days and the date of the bill is April 1st, QuickBooks enters April 10th as the due date.