C++ Date & Time

The C++ standard library does not provide a so-called date type. C++ inherits the C language's structures and functions for date and time operations. In order to use date and time related functions and structures, the <ctime> header file needs to be referenced in the C++ program.

There are four time-related types: clock_t, time_t, size_t and tm. The types clock_t, size_t, and time_t represent the system time and date as certain integers.

Structure typetm Save date and time in the form of C structure. The definition of tm structure is as follows:

struct tm {
  int tm_sec;   // 秒，正常范围从 0 到 59，但允许至 61
  int tm_min;   // 分，范围从 0 到 59
  int tm_hour;  // 小时，范围从 0 到 23
  int tm_mday;  // 一月中的第几天，范围从 1 到 31
  int tm_mon;   // 月，范围从 0 到 11
  int tm_year;  // 自 1900 年起的年数
  int tm_wday;  // 一周中的第几天，范围从 0 到 6，从星期日算起
  int tm_yday;  // 一年中的第几天，范围从 0 到 365，从 1 月 1 日算起
  int tm_isdst; // 夏令时
}

The following is about date and time in C/C++ important functions. All these functions are part of the C/C++ standard library. You can check the details of each function in the C++ standard library.

Current date and time

The following example obtains the current system date and time, including local time and coordinated universal time (UTC).

#include <iostream>
#include <ctime>

using namespace std;

int main( )
{
   // 基于当前系统的当前日期/时间
   time_t now = time(0);
   
   // 把 now 转换为字符串形式
   char* dt = ctime(&now);

   cout << "本地日期和时间：" << dt << endl;

   // 把 now 转换为 tm 结构
   tm *gmtm = gmtime(&now);
   dt = asctime(gmtm);
   cout << "UTC 日期和时间："<< dt << endl;
}

When the above code is compiled and executed, it produces the following results:

本地日期和时间：Sat Jan  8 20:07:41 2011

UTC 日期和时间：Sun Jan  9 03:07:41 2011

Formatting time using structure tm

tm structure It is particularly important when dealing with date and time related operations in C/C++. The tm structure holds the date and time as a C structure. Most time-related functions use the tm structure. The following example uses the tm structure and various date and time related functions.

Before practicing using structures, you need to have a basic understanding of C structures and know how to use the arrow -> operator to access structure members.

#include <iostream>
#include <ctime>

using namespace std;

int main( )
{
   // 基于当前系统的当前日期/时间
   time_t now = time(0);

   cout << "Number of sec since January 1,1970:" << now << endl;

   tm *ltm = localtime(&now);

   // 输出 tm 结构的各个组成部分
   cout << "Year: "<< 1900 + ltm->tm_year << endl;
   cout << "Month: "<< 1 + ltm->tm_mon<< endl;
   cout << "Day: "<<  ltm->tm_mday << endl;
   cout << "Time: "<< 1 + ltm->tm_hour << ":";
   cout << 1 + ltm->tm_min << ":";
   cout << 1 + ltm->tm_sec << endl;
}

When the above code is compiled and executed, it produces the following results:

Number of sec since January 1, 1970:1294548238
Year: 2011
Month: 1
Day: 8
Time: 22: 44:59

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