25 Time library [time]

25.10 Time zones [time.zone]

25.10.8 Class leap [time.zone.leap]

25.10.8.1 Overview [time.zone.leap.overview]

namespace std::chrono {
  class leap {
  public:
    leap(const leap&)            = default;
    leap& operator=(const leap&) = default;

    // unspecified additional constructors

    constexpr sys_seconds date() const noexcept;
  };
}
1
#
Objects of type leap representing the date of the leap second insertions are constructed and stored in the time zone database when initialized.
2
#
[ Example
: 
for (auto& l : get_tzdb().leaps)
  if (l <= 2018y/March/17d)
    cout << l.date() << '\n';
Produces the output:
1972-07-01 00:00:00
1973-01-01 00:00:00
1974-01-01 00:00:00
1975-01-01 00:00:00
1976-01-01 00:00:00
1977-01-01 00:00:00
1978-01-01 00:00:00
1979-01-01 00:00:00
1980-01-01 00:00:00
1981-07-01 00:00:00
1982-07-01 00:00:00
1983-07-01 00:00:00
1985-07-01 00:00:00
1988-01-01 00:00:00
1990-01-01 00:00:00
1991-01-01 00:00:00
1992-07-01 00:00:00
1993-07-01 00:00:00
1994-07-01 00:00:00
1996-01-01 00:00:00
1997-07-01 00:00:00
1999-01-01 00:00:00
2006-01-01 00:00:00
2009-01-01 00:00:00
2012-07-01 00:00:00
2015-07-01 00:00:00
2017-01-01 00:00:00
— end example
 ]