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.
#include <amount.h>
#include <base58.h>
#include <chain.h>
#include <consensus/validation.h>
#include <core_io.h>
#include <httpserver.h>
#include <validation.h>
#include <net.h>
#include <policy/feerate.h>
#include <policy/fees.h>
#include <policy/policy.h>
#include <policy/rbf.h>
#include <rpc/mining.h>
#include <rpc/safemode.h>
#include <rpc/server.h>
#include <script/sign.h>
#include <timedata.h>
#include <util.h>
#include <utilmoneystr.h>
#include <wallet/coincontrol.h>
#include <wallet/feebumper.h>
#include <wallet/wallet.h>
#include <wallet/walletdb.h>
#include <wallet/walletutil.h>
#include <init.h>  
#include <stdint.h>
#include <univalue.h>
static const std::string WALLET_ENDPOINT_BASE = "/wallet/";
CWallet *GetWalletForJSONRPCRequest(const JSONRPCRequest& request)