How to use the freezer.utils.utils.date_to_timestamp function in freezer

def get_start_date(date):
            # start_date format "%Y-%m-%dT%H:%M:%S"
            now = datetime.datetime.now()
            start_date = now + datetime.timedelta(0, 2, 0)
            if (utils.date_to_timestamp(date) >
                    utils.date_to_timestamp(now.isoformat().split('.')[0])):
                start_date = datetime.datetime.strptime(
                    date, "%Y-%m-%dT%H:%M:%S")
            return start_date

def get_backups_from_timestamp(backups, restore_from_timestamp):
                for backup in backups:
                    backup_created_date = backup.created_at.split('.')[0]
                    backup_created_timestamp = (
                        utils.date_to_timestamp(backup_created_date))
                    if backup_created_timestamp >= restore_from_timestamp:
                        yield backup

def get_end_date(start, end):
            # start end format "%Y-%m-%dT%H:%M:%S"
            end_date = datetime.datetime.strptime(end, "%Y-%m-%dT%H:%M:%S")
            if (utils.date_to_timestamp(start) > utils.date_to_timestamp(end)):
                end_date = None
            return end_date
        kwargs_date = {}

def execute(self):
        # remove backups by freezer admin action
        backup_media = self.conf.backup_media
        if backup_media == 'cindernative':
            admin_os = admin.AdminOs(self.conf.client_manager)
            admin_os.del_off_limit_fullbackup(
                self.conf.cindernative_vol_id,
                self.conf.fullbackup_rotation)
            return {}
        if self.conf.remove_from_date:
            timestamp = utils.date_to_timestamp(self.conf.remove_from_date)
        else:
            timestamp = datetime.datetime.now() - \
                datetime.timedelta(days=float(self.conf.remove_older_than))
            timestamp = int(time.mktime(timestamp.timetuple()))

        if self.conf.backup_media == 'cinder':
            if self.conf.cinder_vol_id:
                old_backups = self.get_cinder_old_backups(
                    timestamp,
                    self.conf.cinder_vol_id
                )
                self.remove_backup_dirs(old_backups,
                                        self.conf.cinder_vol_id)
                return {}

            else:

def execute(self):
        conf = self.conf
        LOG.info('Executing Restore...')
        restore_timestamp = None

        restore_abs_path = conf.restore_abs_path
        if conf.restore_from_date:
            restore_timestamp = utils.date_to_timestamp(conf.restore_from_date)
        if conf.backup_media == 'fs':
            self.engine.restore(
                hostname_backup_name=self.conf.hostname_backup_name,
                restore_resource=restore_abs_path,
                overwrite=conf.overwrite,
                recent_to_date=restore_timestamp,
                backup_media=conf.mode)

            try:
                if conf.consistency_checksum:
                    backup_checksum = conf.consistency_checksum
                    restore_checksum = checksum.CheckSum(restore_abs_path,
                                                         ignorelinks=True)
                    if restore_checksum.compare(backup_checksum):
                        LOG.info('Consistency check success.')
                    else: