how do I get the date in a date archive page

Use get_query_var() to get the date parts:

$year     = get_query_var('year');
$monthnum = get_query_var('monthnum');
$day      = get_query_var('day');

In wp_title() a call to get_query_var('m') is used for the month too, but I got always just a 0 as value even on an URL like /2008/09/05/.

If you want to print the month name, use:

$GLOBALS['wp_locale']->get_month($monthnum);

The month name will be translated by WordPress then.

There are also four conditional functions you can use:

is_year()
is_month()
is_day()
is_date()