title: Why is std::chrono::system_clock::to_time_t() not constexpr?
body: <p>The C++ standard (github.com/cplusplus/draft) has the <code>time_t</code> conversion functions (<code>std::chrono::system_clock::to_time_t</code> and <code>std::chrono::system_clock::from_time_t</code>) for listed as <code>static</code> and <code>noexcept</code> but not <code>constexpr</code>.</p>
  
  <p>Given that essentially all of the operations on <code>time_point</code> and <code>duration</code> are <code>constexpr</code> (including <code>duration_cast</code> and <code>time_point_cast</code>), I can't think of any reason to exclude them.  A quick inspection of the libstdc++ sources on my local machine confirms that these functions are implemented as simple duration/time_point casts.</p>
  
  <p>Is there any reason that these two functions should not be <code>constexpr</code>?  Is this just a case of "because no one proposed they should be"?</p>