Use of 'default TEXT now()'

Hi:

I'm writing an application where I have to use a datetime field. In this
field we'll keep the date and time of the coments about some tasks.

I am not sure about the right use of 'default', should I use:

fecha_obs    datetime default TEXT now()

or

fecha_obs    datetime default now() ?

When I must use TEXT ?

I want to retrieve the fields in datetime order.

TIA

                       Roberto Andrade Fonseca
                Universidad Nacional Autónoma de México

                     Teléfonos: 623-1154, 623-1119
               e-mail: randrade@campus.iztacala.unam.mx