title: When is std::chrono epoch?
body: <p><code>std::chrono::time_point::time_since_epoch()</code> returns a <code>duration</code>, referred to some <code>time_point</code> in the past. When is such a <code>time_point</code>? It depends on the C++ implementation or it's defined by the C++ standard? Or it is a de facto standard to set the epoch to 1 January 1970 UTC?</p>