C++中获取日期函数gmtime和localtime区别

悠悠 2023-02-21 06:51 2阅读 0赞

函数gmtime和localtime的声明如下：

struct tm * gmtime (const time_t * timer);
    struct tm * localtime (const time_t * timer);

它们均接收一个time\_t的const指针类型，time\_t类型通常是一个大整数值，该整数值表示自UTC时间1970年1月1日00:00以来经过的秒数即UNIX时间戳，可直接调用time函数获取，如下面测试代码中的time(&rawtime)语句。

它们均返回类型为tm的结构体指针。此结构体的声明如下：由此可见gmtime和localtime函数主要做的就是将大整数值rawtime转换成易读取的时间，如可快速获取年、月、日等。

struct tm {
    	int tm_sec;   // seconds after the minute - [0, 60] including leap second
    	int tm_min;   // minutes after the hour - [0, 59]
    	int tm_hour;  // hours since midnight - [0, 23]
    	int tm_mday;  // day of the month - [1, 31]
    	int tm_mon;   // months since January - [0, 11]
    	int tm_year;  // years since 1900
    	int tm_wday;  // days since Sunday - [0, 6]
    	int tm_yday;  // days since January 1 - [0, 365]
    	int tm_isdst; // daylight savings time flag
    };

gmtime和localtime区别：

(1).gmtime将time\_t转换为UTC时间，UTC的全称为Coordinated Universal Time，即世界标准时间。

(2).localtime将time\_t转换为本地时间(local time)。北京时间比UTC时间早8小时。

测试代码段如下：

void print_time(const std::string& type, const struct tm* timeinfo)
    {
    	int year = timeinfo->tm_year + 1900; // years since 1900
    	int month = timeinfo->tm_mon + 1; // months since January - [0, 11]
    	int day = timeinfo->tm_mday;
    	int date = year * 10000 + month * 100 + day;
    
    	int hour = timeinfo->tm_hour;
    	int minute = timeinfo->tm_min;
    	int second = timeinfo->tm_sec;
    
    	fprintf(stdout, "type: %s\t, date: %d, time: %.2d:%.2d:%.2d\n", type.c_str(), date, hour, minute, second);
    }
    
    int test_gmtime_localtime()
    {
    { // gmtime
    	time_t rawtime;
    	time(&rawtime);
    
    	struct tm* timeinfo = gmtime(&rawtime);
    	print_time("UTC", timeinfo);
    }
    
    { // localtime
    	time_t rawtime;
    	time(&rawtime);
    
    	struct tm* timeinfo = localtime(&rawtime);
    	print_time("localtime", timeinfo);
    }
    
    	return 0;
    }

执行结果如下：localtime比UTC早8小时

![20200629165516756.png][]

**GitHub**：[https://github.com/fengbingchun/Messy\_Test][https_github.com_fengbingchun_Messy_Test]

[20200629165516756.png]: https://img-blog.csdnimg.cn/20200629165516756.png
[https_github.com_fengbingchun_Messy_Test]: https://github.com/fengbingchun/Messy_Test