If I unlock an encrypted wallet for 359711509437336256 or less seconds, I can sign transactions with it:

    $ bitcoin-cli walletpassphrase "$x" 359711509437336256
    $ echo $?
    0
    $ bitcoin-cli signrawtransaction $tx | grep complete
      "complete": true

But if I unlock it for 359711509437336257 or more seconds, it appears to have unlocked but I can't sign any transactions:

    $ bitcoin-cli walletpassphrase "$x" 359711509437336257
    $ echo $?
    0
    $ bitcoin-cli signrawtransaction $tx | grep complete
    error code: -13
    error message:
    Error: Please enter the wallet passphrase with walletpassphrase first.

I don't know the significance of that number. It's nowhere near a power of 2.

I was trying to unlock the wallet effectively "forever", so put a big number. If I use too big a number (2^63 or more) I get an error message `JSON integer out of range` but for numbers between 359711509437336257 and 2^63-1 inclusive I get no error, but no actual unlocking.
}
    pwallet->Lock();
    pwallet->nRelockTime = 0;
    return NullUniValue;
}
UniValue encryptwallet(const JSONRPCRequest& request)
{
    CWallet * const pwallet = GetWalletForJSONRPCRequest(request);
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() != 1) {
        throw std::runtime_error(
            "encryptwallet \"passphrase\"\n"
            "\nEncrypts the wallet with 'passphrase'. This is for first time encryption.\n"
            "After this, any calls that interact with private keys such as sending or signing \n"
            "will require the passphrase to be set prior the making these calls.\n"
            "Use the walletpassphrase call for this, and then walletlock call.\n"
            "If the wallet is already encrypted, use the walletpassphrasechange call.\n"
            "Note that this will shutdown the server.\n"
            "\nArguments:\n"
            "1. \"passphrase\"    (string) The pass phrase to encrypt the wallet with. It must be at least 1 character, but should be long.\n"
            "\nExamples:\n"
            "\nEncrypt your wallet\n"
            + HelpExampleCli("encryptwallet", "\"my pass phrase\"") +
            "\nNow set the passphrase to use the wallet, such as for signing or sending bitcoin\n"
            + HelpExampleCli("walletpassphrase", "\"my pass phrase\"") +
            "\nNow we can do something like sign\n"
            + HelpExampleCli("signmessage", "\"address\" \"test message\"") +
            "\nNow lock the wallet again by removing the passphrase\n"