If you hit z to goto stocks, you can see what the date is. I'd love to have the date always show up at the top. I've skimmed through the init file and don't see anything likely.
The date is not currently something that can be displayed. Though I do not know of any currently, I think it should be possible to display the date via a mod -- perhaps you should try asking on the Modding section of the bay12 forums?
Do note, that the date is also mentioned for any given Announcement, if you're trying to determine when something happened.