[web2py] NameError: name 'now' is not defined

[Sciurus vulgaris]

Hello,

i got an exception when using the sample code for storring the sessions in 
a database. I used the sample code from 
http://www.web2py.com/book/default/chapter/04#session

Model:
...
db.define_table('web2py_session',
    Field('locked', 'boolean', default=False),
    Field('client_ip'),
    Field('created_datetime', 'datetime', default=now),
    Field('modified_datetime', 'datetime'),
    Field('unique_key'),
    Field('session_data', 'text'))
...

Exception:

Traceback (most recent call last):
  File "/home/geht/euch/nichts/an/web2py/gluon/restricted.py", line 192, in 
restricted
    exec ccode in environment
  File 
"/home/geht/euch/nichts/an/web2py/applications/moepse/models/default.py", line 
30, in <module>
    Field('created_datetime', 'datetime', default=now),
NameError: name 'now' is not defined


Kind Regards,

Sciurus vulgaris