#include ＜ctime＞

C++ 标准库提供了丰富的功能，其中 <ctime> 是处理时间和日期的标准库之一。它提供了一组函数，用于获取当前时间、日期以及执行时间相关的计算。

// -*- C++ -*- forwarding header.

// Copyright (C) 1997-2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/ctime
 *  This is a Standard C++ Library file.  You should @c \#include this file
 *  in your programs, rather than any of the @a *.h implementation files.
 *
 *  This is the C++ version of the Standard C Library header @c time.h,
 *  and its contents are (mostly) the same as that header, but are all
 *  contained in the namespace @c std (except for names which are defined
 *  as macros in C).
 */

//
// ISO C++ 14882: 20.5  Date and time
//

#pragma GCC system_header

#include <bits/c++config.h>
#include <time.h>

#ifndef _GLIBCXX_CTIME
#define _GLIBCXX_CTIME 1

// Get rid of those macros defined in <time.h> in lieu of real functions.
#undef clock
#undef difftime
#undef mktime
#undef time
#undef asctime
#undef ctime
#undef gmtime
#undef localtime
#undef strftime

namespace std
{
  using ::clock_t;
  using ::time_t;
  using ::tm;

  using ::clock;
  using ::difftime;
  using ::mktime;
  using ::time;
  using ::asctime;
  using ::ctime;
  using ::gmtime;
  using ::localtime;
  using ::strftime;
} // namespace

#if __cplusplus >= 201703L && defined(_GLIBCXX_HAVE_TIMESPEC_GET)
#undef timespec_get
namespace std
{
  using ::timespec;
  using ::timespec_get;
} // namespace std
#endif

#endif

以下是 <ctime> 库中一些常用函数的基本语法：

• 获取当前时间（以秒为单位，从1970年1月1日开始计算）：

  time_t t = time(NULL);

• 将 time_t 类型的时间转换为 tm 结构体：

  struct tm *tm = localtime(&t);

• 将 time_t 类型的时间转换为协调世界时（UTC）的 tm 结构体：

  struct tm *tm_utc = gmtime(&t);

• 格式化时间：

  char buffer[80];
  strftime(buffer, 80, "%Y-%m-%d %H:%M:%S", tm);