When creating a new invoice on a windows installation of Gnucash, the 'date opened' and invoice dates default to the first day of the current month and cannot be successfully changed.

Steps to reproduce:
1. Select Business->Customer->New invoice; the Invoice popup dlg appears
2. Note the 'date opened' defaults to the first of the month
3. Edit date opened via the calendar control to the desired date
4. Edit the other fields as desired
5. Press Ok; an invoice tab window appears
6. Note the 'date opened' field is back to the first of the month, and can no longer be edited
7. Note the invoice date in the invoice record defaults to the current date.
8. Enter the desired date in the invoice record
9. Fill in the rest of the invoice record as desired.
10. Post the invoice, choose the A/R account, etc.
11. Open the A/R acct register: note the transaction date for the posted invoice is the first of the month, not the date you entered in the invoice record.

This has been observed on Windows XP SP3. It does not happen on OS X 10.4.11