Defined in header <chrono>
struct last_spec
    explicit last_spec() = default;
(since C++20)
inline constexpr last_spec last{};
(since C++20)

last_spec is an empty tag type that is used in conjunction with other calendar types to indicate the last thing in a sequence. Depending on context, it may indicate the last day of a month (as in 2018y/February/last, for last day of February 2018, i.e., 2018-02-28) or the last day of the week in a month (as in 2018/February/Sunday[last], for last Sunday of February 2018, i.e., 2018-02-25).

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