When read_status set to finish, add a date to read_at

Hey ho Folks,

i have a read_status field (Text, but with a fixed set of permitted values) with the values "finished, read at moment, unread, wishlist". And a field read_at (Date).

What I want to achieve is when I change the read_status to "finished", than the field read_at would automatically get the current date.

Is this possible? Or did I have the only choice to make it with hand?

Thanks already, and greetz.