If you are talking about expiration date, I prefer not to put one. I know all the ad gurus say you need to create a sense of urgency, but some people are just not ready to clean right then and they will hang onto your postcard. I get postcards 11 months later sometimes and what it does is increases my return on investment. I get 5:1 return on my mailings but I doubt I could if they expired in 30 days.
However, if I am mailing a really good wintertime special I usually have it expire in March right before the busy season.