This detail may sound silly but it’s very important. If we used clocks and timestamps to set our transactions in order, we need to trust someone with the responsibility to keep that clock in time. And that centralized actor could literally turn back time and make transactions never happen in our lists. This sounds like sci-fi, but that’s only because we’re used in our regular life to all of the clocks around us to be more or less in sync.
Let’s make a thought experiment: if in the middle of the day, all the clocks at your disposal showed that it’s 3 in the afternoon when it’s actually 5. Would you be able to notice it? Would you trust yourself or the clocks? Even if you realize that the hour was changed, how could you prove it to someone else?
In the case of the real world and clocks, we have a decentralized set of more than 400 atomic clocks in 80 different institutions that all contribute to an average called the International Atomic Time. It would be difficult for all those institutes to conspire to mess up our clocks worldwide.
In the case of a cryptocurrency like Monero, we need a clock that is self-regulating, independent, and cannot be taken backwards. Specially not without us noticing. And even better, if it happens, anybody can prove easily that something is wrong.
The clock we’re using are the blocks of our blockchain and new block is the Monero smallest unit of time. Every block is a tick of Monero’s clock. One after another, linked.