A good way of storing and working with dates and time in C++.  A structure containing a calendar date and time broken down into its components.

Creating a tm

#include <ctime>
	
	tm my_date_time;

	my_date_time.tm_year = (2015 - 1900);	//years since 1900 (we must be >= 1970 for time_t based calcs)
	my_date_time.tm_mon = 1;		//months since January (0-11)
	my_date_time.tm_mday = 18;
	my_date_time.tm_hour = 12;
	my_date_time.tm_min = 03;
	my_date_time.tm_sec = 48;

Note:

tm_year is years since 1900!!
tm_month is months since January (0-11)!!
Note when converting to time_t your year must be >= 1970 as time_t is usually based from 00:00, Jan 1 1970 UTC!

Loading with current time

#include <time.h>
#include <sys/time.h>

	timeval curTime;
	tm *my_date_time;
	gettimeofday(&curTime, NULL);
	my_date_time = localtime(&curTime.tv_sec);
	 = my_date_time->tm_year - 100;
	 = my_date_time->tm_mon + 1;
	 = my_date_time->tm_mday;
	 = my_date_time->tm_hour;
	 = my_date_time->tm_min;
	 = my_date_time->tm_sec;

Converting tm to Universal Time String

	char my_date_time_string[22];
	strftime(my_date_time_string, sizeof(my_date_time_string), "%Y-%m-%dT%H:%M:%S", &my_date_time);

Convert tm to StringStream

	//Create as yyyymmddhh
	stringstream ss_archive_file_timestamp;
	ss_archive_file_timestamp << (tmFromDate.tm_year + 1900);							//2000 - 2099
	ss_archive_file_timestamp << setfill('0') << setw(2) << (tmFromDate.tm_mon + 1);	//1-12
	ss_archive_file_timestamp << setfill('0') << setw(2) << tmFromDate.tm_mday;			//1-31
	ss_archive_file_timestamp << setfill('0') << setw(2) << tmFromDate.tm_hour;			//0-23

	//Create in UTC format
	stringstream ss_archive_file_timestamp_utc;
	ss_archive_file_timestamp_utc << (tmFromDate.tm_year + 1900) << "-";							//2000 - 2099
	ss_archive_file_timestamp_utc << setfill('0') << setw(2) << (tmFromDate.tm_mon + 1) << "-";		//1-12
	ss_archive_file_timestamp_utc << setfill('0') << setw(2) << tmFromDate.tm_mday << " ";			//1-31
	ss_archive_file_timestamp_utc << setfill('0') << setw(2) << tmFromDate.tm_hour << ":00:00";		//0-23

Convert String To DateTime

	std::tm tm1 = {};
	if (strptime(MyDateCharString, "%Y-%m-%d", &tm1) == NULL)    //Can include time characters too if wanted "%Y-%m-%d %H:%M:%S"
	{
		//Failed

	}

Compare tm’s

	if (mktime(&tm1) > mktime(&tm2))		//Larger values of mktime() are more recent in time
	{
	}

Add, Subtract with tm’s

https://raspberry-projects.com/pi/programming-in-c/datetime/datetime-calculations