PyMongo, handling fields with time relative to the database time

Say I have a document field name "creation_time". I would like to have its value initialized with the database current time, instead using the client machine time with:

datetime.datetime.utcnow()

How can I achieve this with PyMongo?

Moreover is there any way to do operations during the field initialization, for example:

"creation_time": magic_get_mongodb_time() + 10 hours

As a side note I'm OK to handle unix timestamps, not necessary datetime format.

EDIT: operations on dates are not possible yet (May 2015): http://ift.tt/1FTC07K