This is a wizard that is located on stock/wizard/stock_invoice_onshipping.

You can inherit it with this:

from datetime import date
from openerp.osv import fields, osv
from openerp.tools.translate import _

class stock_invoice_onshipping(osv.osv_memory):

    _inherit = "stock.invoice.onshipping"

    _defaults={
        invoice_date': date.today().strftime('%Y-%m-%d'),
    }

Or you can add that line on defaults to the original file, but this is no recommended because if you want to update your OpenERP version you have to check all your changes and modify them on the new version. do this as a new module and it's going to work.