« I had 8pm in mind, but I’m not sure at all » – you may want to correct it to: « I presume it might start at 8pm [though I could be wrong.] » By saying you « had 8pm in mind » a few times already, you gave the impression that you were actually so sure/privy to insider’s information that you come across as cocky. Then in this latest reply you added « I’m not sure at all » as a disclaimer – you’re backpedalling. Which not only leaves the casual English native speaker confused, but you being unreliable.