How do I do the equivalent of this SQL in django?

```
UPDATE table SET timestamp=NOW() WHERE ...

```

Particularly I want to set the datetime field using server's builtin function to get the system time from the server that the database was running on and not the time on the client machine. 

I know you can execute the raw sql directly but I'm looking for a more portable solution since databases have different functions for getting the current datetime. 

Edit: few people mentioned auto\_now param. This updates the datetime on every modification while I want to update datetime only on certain occasions.