Mostly for converting between std::time_t and broken-down time
stamps. Uses UTC and std::chrono::system_clock. May wrap in as
little as 292 years, depending on the period of system_clock.
Intended to replace various timestamping utility functions from
libecl. A comprehensive time-service protocol for Flow is much more
work than this, and will likely not be easily realized before we
have C++17 and its much expanded time/calendar library.
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