Cplusplus 简明教程

C++ Date and Time

C 标准库不提供适当的日期类型。C 继承自 C 的用于日期和时间操作的结构和函数。要访问日期和时间相关的函数和结构，你需要在 C++ 程序中包含 <ctime> 头文件。

The C standard library does not provide a proper date type. C inherits the structs and functions for date and time manipulation from C. To access date and time related functions and structures, you would need to include <ctime> header file in your C++ program.

有四种与时间相关的类型： clock_t, time_t, size_t ，和 tm 。类型为 clock_t、size_t 和 time_t 的变量能够表示某种形式的整数系统时间和日期。

There are four time-related types: clock_t, time_t, size_t, and tm. The types - clock_t, size_t and time_t are capable of representing the system time and date as some sort of integer.

结构类型 tm 以 C 结构的形式保存日期和时间，该结构包含以下元素：

The structure type tm holds the date and time in the form of a C structure having the following elements −

struct tm {
   int tm_sec;   // seconds of minutes from 0 to 61
   int tm_min;   // minutes of hour from 0 to 59
   int tm_hour;  // hours of day from 0 to 24
   int tm_mday;  // day of month from 1 to 31
   int tm_mon;   // month of year from 0 to 11
   int tm_year;  // year since 1900
   int tm_wday;  // days since sunday
   int tm_yday;  // days since January 1st
   int tm_isdst; // hours of daylight savings time
}

以下是在C或C中处理日期和时间时使用的一些重要函数。所有这些函数都是C和C库的标准部分，你可以使用下面给出的C++标准库的引用来检查它们的详细信息。

Following are the important functions, which we use while working with date and time in C or C. All these functions are part of standard C and C library and you can check their detail using reference to C++ standard library given below.

Sr.No

Function & Purpose

1

time_t time(time_t *time); This returns the current calendar time of the system in number of seconds elapsed since January 1, 1970. If the system has no time, .1 is returned.

2

char *ctime(const time_t *time); This returns a pointer to a string of the form day month year hours:minutes:seconds year\n\0.

3

struct tm *localtime(const time_t *time); This returns a pointer to the tm structure representing local time.

4

clock_t clock(void); This returns a value that approximates the amount of time the calling program has been running. A value of .1 is returned if the time is not available.

5

char * asctime ( const struct tm * time ); This returns a pointer to a string that contains the information stored in the structure pointed to by time converted into the form: day month date hours:minutes:seconds year\n\0

6

struct tm *gmtime(const time_t *time); This returns a pointer to the time in the form of a tm structure. The time is represented in Coordinated Universal Time (UTC), which is essentially Greenwich Mean Time (GMT).

7

time_t mktime(struct tm *time); This returns the calendar-time equivalent of the time found in the structure pointed to by time.

8

double difftime ( time_t time2, time_t time1 ); This function calculates the difference in seconds between time1 and time2.

9

size_t strftime(); This function can be used to format date and time in a specific format.

Current Date and Time

假设您想要获取当前的系统日期和时间，无论是作为本地时间还是协调世界时 (UTC)。以下示例可实现此目标 −

Suppose you want to retrieve the current system date and time, either as a local time or as a Coordinated Universal Time (UTC). Following is the example to achieve the same −

#include <iostream>
#include <ctime>

using namespace std;

int main() {
   // current date/time based on current system
   time_t now = time(0);

   // convert now to string form
   char* dt = ctime(&now);

   cout << "The local date and time is: " << dt << endl;

   // convert now to tm struct for UTC
   tm *gmtm = gmtime(&now);
   dt = asctime(gmtm);
   cout << "The UTC date and time is:"<< dt << endl;
}

编译并执行上述代码后，将产生以下结果 −

When the above code is compiled and executed, it produces the following result −

The local date and time is: Sat Jan  8 20:07:41 2011

The UTC date and time is:Sun Jan  9 03:07:41 2011

Format Time using struct tm

tm 结构无论在 C 还是 C++ 中，在处理日期和时间时都非常重要。此结构以 C 结构的形式保存日期和时间，如上所述。大多数时间相关函数会使用 tm 结构。以下是一个示例，展示了如何使用各种日期和时间相关函数以及 tm 结构 −

The tm structure is very important while working with date and time in either C or C++. This structure holds the date and time in the form of a C structure as mentioned above. Most of the time related functions makes use of tm structure. Following is an example which makes use of various date and time related functions and tm structure −

在此章节中使用结构时，我假设您基本了解 C 结构以及如何使用箭头 → 操作符访问结构成员。

While using structure in this chapter, I’m making an assumption that you have basic understanding on C structure and how to access structure members using arrow → operator.

#include <iostream>
#include <ctime>

using namespace std;

int main() {
   // current date/time based on current system
   time_t now = time(0);

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

   tm *ltm = localtime(&now);

   // print various components of tm structure.
   cout << "Year:" << 1900 + ltm->tm_year<<endl;
   cout << "Month: "<< 1 + ltm->tm_mon<< endl;
   cout << "Day: "<< ltm->tm_mday << endl;
   cout << "Time: "<< 5+ltm->tm_hour << ":";
   cout << 30+ltm->tm_min << ":";
   cout << ltm->tm_sec << endl;
}

编译并执行上述代码后，将产生以下结果 −

When the above code is compiled and executed, it produces the following result −

Number of sec since January 1,1970 is:: 1588485717
Year:2020
Month: 5
Day: 3
Time: 11:31:57