Using stored functions in Active Record

[eluser]Alexxz[/eluser]
How to use some DB functions in Active record?
For example in MySQL,

Code:

UPDATE table SET value = 'new value', changed = NOW();

How can I produce that kind of query with active record?
May be there is some dirty hack?
I can't use php's time functions because i have a lot of servers with different time and one DB.