std::chrono::month:: month

From cppreference.com
Utilities library
General utilities
Relational operators (deprecated in C++20)
month ( ) = default ;
(1) (since C++20)
constexpr explicit month ( unsigned m ) noexcept ;
(2) (since C++20)

Constructs a month object.

1) Default constructor leaves the month value uninitialized.
2) If m <= 255 , constructs a month object holding the month value m . Otherwise the value held is unspecified.