Non-disclosure agreements in the U.S. are enforceable. Assuming they are properly written to provide protection for all information which the parties wish to protect. However, non-disclosure agreements, like other types of contracts, can only protect the information disclosed AFTER they are signed. One cannot obligate a party to keep quiet about something that party has already learned about after the fact. It may be possible, if "valuable consideration" is exchanged after the fact, for a party to enter into a non-disclosure agreement with another in which that party then agrees to maintain the confidentiality of information learned previously, however, any disclosure that may have been made prior to that signature may not make that person liable. "Valuable consideration" is a contract term and no contract is binding or enforceable without consideration. Consideration normally means an exchange of something of value (such as money, property, or even promises) that serves to establish the parties to a contract both received or gave something to obtain the benefit of the contract. However, where parties refuse to sign a contract there is no action that can force them to do so.
When drafting a contract, particularly one that may try to go into a matter after the fact, the language used must be precise and is critically important. For these reasons legal counsel should be hired, in your state, to assure any such contract will be binding and to assure any consideration exchanged would be held valid by state courts.
Ideas, without agreements to protect them, are not provided protection under intellectual property laws (so timelines may not matter in such a case). If a disclosure is made about something other than an idea, such as a full product, a book, etc. then that may be protectable. In such a case it may be good to have a list of contacts, disclosure, etc., but, in such cases, it may be better to work with an attorney to be sure proper intellectual property filings, if applicable, are made first with the copyright or patent and trademark offices as being first to file is often the best protection for anything which intellectual property laws protect.
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