I'll explain this with a use case....

I need to get our developers to submit a 'Status Report' on a particular issue type (normally once a week). I've created a workflow transition called 'Report Status' which brings up a reporting screen (works well). The 'Status Report' is a free text custom field. Whenever they update the 'Status Report' via the transition+screen I would like to set a custom date field called 'Status Report Date' to $TODAY$.

This will record when the last 'Status Report' was submitted.

Thanks.

p.s. sorry if this is a duplicate.

ALTERNATIVELY.....
I can't validate that they have updated 'Status Report Date'==TODAY manually either.