I'm currently working on a quizz app. When a question appears the users has 10 seconds to answer it, otherwise they don't get the points for that question. Once the timer is up, I want to autmatically move to the next question. I am currently facing issues on how to make the 10 second countdown timer "unhackable" by the client.

My initial idea was to use something along the lines of setTimeout() on the client-side for 10 seconds, and once the timer is complete, ask the server to fetch the next question. The problem with this is that the client-side timer can be hacked/modified to run for longer than 10 seconds, potentionally giving some users longer than 10 seconds to answer the question.

client <--- sends question --- server
  |
start timer for 10 seconds (as this is client-side, it could easily be extended)
  |
  .
10 seconds later 
  .
  V
client --- ask for next question / send answer ---> server

In order to keep it unhackable, I thought of moving the time checking logic to the sever-side. This would involve keeping two variables (A and B) on the server-side per connected user, one representing the time the question was sent, and the other representing the time an answer was given. The client-side timer would still run, except the server side uses the time-stamps to perform some validation to check if the difference between the timestamps A and B exceeds 10 seconds:

client <--- sends question --- server (send question at timestamp `A`)
  |
start timer for 10 seconds (as this is client-side, it could easily be extended)
  |
  .
10 seconds later 
  .
  V
client --- ask for next question / send answer ---> server (receive request at timestamp `B`)
                                                      |
+-----------------------------------------------------+
v
server logic:
  duration = B - A
  if(duration > 10 seconds) {
    // allocated time exceeded
  }

However, I see a few potentional flaws with this. The time it takes for the question to arrive to the client from the server and the time between when the server sent the questioon (time A) to the time that the client-side timer starts won't be instentationous and will depend on the ping / connection that the user has to the server. Similar ping issues exist when the client asks for the next question. Moreover, I'm worried that if the client-side timer which is supposed to run for 10 seconds lags behind a little, then it would also cause the server-side check to fail. As a result, checking if the duration exceeded 10 seconds isn't enough, and it would require some additional buffer. However, I feel like arbitarly hard-coding the buffer to something like 1 or 2 seconds could potentionally still lead to issues and feels like a bit of a hacky work-around that isn't very robust.

Question: I'm wondering if there is a different approach that I am missing to keep the client side timer unhackable and accurate. I also want to try and avoid creating seperate timers with setTimeout() or alike for each connected user on the server-side, as many users could be connected at one given point in time, and having so many timers queued up on the server feels unresourceful. I also want to try and keep the number of messages sent back and forth between the client and the server to a minimum.