Http Session timeout clock

Hello all,
Need some helps on a couple of questions regarding http session timeout clock, thanks in advance

1) Am I correct to say that the time-out clock of a http session starts when the Http Session becomes inactive (passivated)?

2) I'm assuming that a session timeout clock is cleared when requester submits a http post to the server. Does the timeout clock starts again as soon as the server received the post or does it starts right after it returns the respond back to the requester?

Thanks