[Ethereum] Is the block.timestamp value in Solidity seconds or milliseconds

I believe the block.timestamp is the wall-clock date/time at which the block was mined. A block number set in the future based on some other block is based on an estimate of the rate of block mining. For example, the average/target block time is, at the time of writing, about ~17s. So if you want to set a time 24 hours in the future via block number, you can use 24 * 60 * 60 / 17 (number of seconds in a day divided by number of seconds per block) to determine roughly what block number will be mined at that time. Note that for times in the mid- and far- future, you will need to take into account the ice age: the rate of block generation will slow down, by design, to encourage a hard fork for proof-of-stake and, in the meantime, makes the profitability of mining decrease materially (when denominated in eth) as the ice age progresses.

Here is a simulator to get an idea of what is happening: https://gist.github.com/CJentzsch/c78768f9837afb8eef74 Note that the rate of slowing has been slowed by a more recent change to the Ethereum protocol, so the simulator is out of date.

The block.timestamp should indicate approximately when the block was mined. But it is only an approximation, it depends on the precision of the clock of the machines working on it, also a miner can modify it. So it is not a very secure measure of the time but for your use case should be good.

From solidity documentation

block.timestamp (uint): current block timestamp as seconds since unix epoch.

You can do something like this:

mapping(address => uint) startTime;

function start() {
    startTime[msg.sender] = now;
}

function hasElapsed() constant return (bool) {
    if (now >= startTime[msg.sender] + 60 minutes) {
        // 60 minutes has elapsed from startTime[msg.sender]
        return true;
    }
    return false;
}