datetime - convert between TAI labels and seconds

          #include <datetime.h>

          void datetime_tai(&dt,t);

          datetime_sec datetime_untai(&dt);

          struct datetime dt;
          datetime_sec t;

          International Atomic Time, TAI, is the fundamental unit for
          time measurements.  TAI has one label for every second of
          real time, without complications such as leap seconds.

          A struct datetime variable, such as dt, stores a TAI label.
          dt.year is the year number minus 1900; dt.mon is the month
          number, from 0 (January) through 11 (December); dt.mday is
          the day of the month, from 1 through 31; dt.hour is the
          hour, from 0 through 23; dt.min is the minute, from 0
          through 59; dt.sec is the second, from 0 through 59; dt.wday
          is the day of the week, from 0 (Sunday) through 6
          (Saturday); dt.yday is the day of the year, from 0 through

          The datetime library supports more convenient TAI
          manipulation with the datetime_sec type.  A datetime_sec
          value, such as t, is an integer referring to the tth second
          after the beginning of 1970 TAI.  The first second of 1970
          TAI was 0; the next second was 1; the last second of 1969
          TAI was -1.  The difference between two datetime_sec values
          is a number of real-time seconds.

          datetime_tai converts a datetime_sec to a TAI label.

          datetime_untai reads a TAI label (specifically dt.year,
          dt.mon, dt.mday, dt.hour, dt.min, and dt.sec) and returns a


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