You can use following method as reference. This is available in addons/hr_holidays/hr_holidays.py(Line 262 in odoo8)
    
    def _get_number_of_days(self, date_from, date_to):
        """Returns a float equals to the timedelta between two dates given as string."""

        DATETIME_FORMAT = "%Y-%m-%d %H:%M:%S"
        from_dt = datetime.datetime.strptime(date_from, DATETIME_FORMAT)
        to_dt = datetime.datetime.strptime(date_to, DATETIME_FORMAT)
        timedelta = to_dt - from_dt
        diff_day = timedelta.days + float(timedelta.seconds) / 86400
        return diff_day

Hope this will helpful.