I need to implement the following:

1. User input user id and pass
2. We validate that on another server
3. If they are correct, cookies with these details should be saved for one month
4. Each time user uses my site, we should look for cookies
5. If they are not found - go to step 1

How can I set cookies for 1 month?

Will the following work?

self.response.headers.add_header('Set-Cookie', 'credentials=%s; expires=Fri, 31-Dec-2020 23:59:59 GMT' \% credentials.encode())

How to calculate one month from now in the required format?

You can use webapp.Response.set_cookie() method:

import datetimeself.response.set_cookie('name', 'value', expires=datetime.datetime.now(), path='/', domain='example.com')

Formatting dates for cookies is something like this:

print (datetime.datetime.now() + datetime.timedelta(weeks=4)).strftime('%a, %d %b %Y %H:%M:%S GMT')