Hello Zahin

You can use the field 'create_date', this is the "magic" column of openerp which is managed by openerp itself, so you will not need to put any default value for this and system will automatically put value in it.

create_date=fields.datetime("Log Date")

OR

create a function which returns date

    def _get_default_date(self,cr,uid,context={}):     
        return fields.date.context_today('your.model.name',cr,uid,context=context)

_defaults={'date':_get_default_date}

Hope this helps !!!

I also learned it from the hard way that when _defaults evaluate time directly it will evaluate it on server start ONLY.  Use fields.date.today() and fields.date.now() instead (see odoo/openerp/osv/fields.py).