Workflow FIeld Update

Hi,

I need to create a workflow that updates a field with a current timestamp when the sale owner of a field is changed.

Last modified does not apply as the record with be modified several times outside of the sale owner change.

Is it possible for a workflow to update a field with a timestamp, rather than just a text string?

If not, can you suggest other ways that I could update a field with "First Modified" timestamp.

Thanks,
Brendan