Even Google&#x27;s &quot;now&quot; is still a little fuzzy -- their system doesn&#x27;t so much define &quot;now&quot; as it defines the uncertainty around &quot;now&quot;, which you can take advantage of for figuring out ordering. This paragraph (from section 1 of their paper) summarizes it pretty well:<p>&gt; The key enabler of these properties is a new TrueTime
&gt; API and its implementation. The API directly exposes
&gt; clock uncertainty, and the guarantees on Spanner’s timestamps
&gt; depend on the bounds that the implementation provides.
&gt; If the uncertainty is large, Spanner slows down to
&gt; wait out that uncertainty.<p>...so really, GPS and atomic clocks aren&#x27;t for defining &quot;now&quot; they&#x27;re for providing a keeping the window of uncertainty around &quot;now&quot; to a small value. The value mentioned in the paper for this is &quot;generally less than 10ms&quot;, which is pretty good for a large-scale distributed system, but massive in terms of precision timekeeping. It is, after all, an uncertainty that&#x27;s three orders of magnitude larger than the GPS glitch that started this discussion...