The problem with atomic clocks in general is that they have very good short term stability, but not-so-great long-term stability. So they are very good at measuring the amount of time that has passed between two events, but not as great for telling you exactly what time an event happened.<p>The other problem is that there&#x27;s not really such thing as a single, true time. Precision timekeeping is one of those places where relativity matters -- differences in altitude and the geology of the earth under your feet at a particular point change how fast any particular clock &#x27;ticks&#x27;.<p>So to accommodate these differences, if you want events to be synchronized across multiple locations over a long period, you both need a single agreed-upon &quot;current time&quot;, and some way to get information about that time information to the sites that need it.<p>You could surely do this with directly connected wires that were very carefully measured, but that gets expensive very quickly. The GPS system already requires super-precise time just to function, so it makes for a great distribution mechanism of &quot;what time is it&quot; information, where the people running the GPS system are bothered with all the details of figuring out and standardizing a single time, and you &#x27;just&#x27; have to listen.<p>The GPS folks also keep track of things like how fast the GPS standard clock is drifting from the UTC standard clock (since no two clocks will agree over the long term, period). And this is actually where the problem happened -- Two of the parameters broadcast (A0 and A1) were incorrect, and it&#x27;s these parameters that are used to specify the current time difference between the GPS master clock and the UTC master clock (the GPS master clock is disciplined to be as close to UTC as possible, but is always off by a minute amount, and that&#x27;s what A0&#x2F;A1 represent).<p>Pretty much it boils down to, much as it is in distributed systems, &quot;there is no now&quot;. Even with maximum care and the highest quality clocks, there&#x27;s really not a definition of &#x27;now&#x27; that&#x27;s consistent across more than a single location. GPS is currently the closest thing we have to a globally-distributed definition of &#x27;now&#x27;.