Jeff, the sale is for any one item. Are you saying you don't know if the one item you're looking for has been released in the last 30 days?
In the case of MTH, you can simply check the MTH website. Every item description shows the month and year it was delivered to dealers.
Lionel's website isn't as specific but it does show which catalog the item appeared in. Also, since Lionel changed their numbering system, anything 6-##### is old by now. Good luck.