Time difference in C++ // not dependent on local time, system time // neutral // UTC 0

Question

Hello,

I get a time from a database. I have to compare this with my time. Is smaller than 30 seconds -> Error message, is greather than all fine:-) Output the difference in seconds. How can I best implement this?

string strHour = "19";
string strMinute = "05";
string strSecond = "44";
string strYear = "2024";
string strMonth = "01";
string strDay = "8";

What is the best?

 time_t currentTime;
 struct tm *localTime;

 time( &currentTime );                   // Get the current time
 localTime = localtime( &currentTime );  // Convert the current time to the local time

 int Hour   = localTime->tm_hour;
 int Min    = localTime->tm_min;
 int Sec    = localTime->tm_sec;
//*******      OR
// To get the time expressed as UTC   GetSystemTime  ?
SYSTEMTIME st;
GetSystemTime(&st);

C++, what do I need to know about the UTC time. What formatting?

Answer 1

Hi, @Markus Freitag Refer to the document: Local Time To convert a local time to a UTC-based time, use the TzSpecificLocalTimeToSystemTime function.

//convert loacl tm to UTC 

	SYSTEMTIME UtcTm;

	tm temptm;

	localtime_s(&temptm, &currentTm);

	SYSTEMTIME localTm = { 1900 + temptm.tm_year,

		1 + temptm.tm_mon,
		temptm.tm_wday,
		temptm.tm_mday,
		temptm.tm_hour,
		temptm.tm_min,
		temptm.tm_sec,
		0 };


	TzSpecificLocalTimeToSystemTime(&tmInfo, &localTm, &UtcTm);
	struct tm tempUTC = { UtcTm.wSecond, UtcTm.wMinute, UtcTm.wHour, UtcTm.wDay, UtcTm.wMonth - 1, UtcTm.wYear - 1900, UtcTm.wDayOfWeek, 0, 0 };
	time_t utctm_t = mktime(&tempUTC);

	// Calculate the time difference in seconds
	auto difference = std::difftime(utctm_t, std::mktime(&dbTime));

Console sample:

#include <iostream>
#include <chrono>
#include <ctime>
#include<string>
#include<Windows.h>

int main() {

	// Database time

	TIME_ZONE_INFORMATION tmInfo;
	GetTimeZoneInformation(&tmInfo);

	std::tm dbTime = {};

	dbTime.tm_year = std::stoi("2024") - 1900;
	dbTime.tm_mon = std::stoi("01") - 1;
	dbTime.tm_mday = std::stoi("11");
	dbTime.tm_hour = std::stoi("9");
	dbTime.tm_min = std::stoi("03");
	dbTime.tm_sec = std::stoi("00");


	// Current time

	auto currentTime = std::chrono::system_clock::now();
	auto currentTm = std::chrono::system_clock::to_time_t(currentTime);



	//convert loacl tm to UTC 

	SYSTEMTIME UtcTm;

	tm temptm;

	localtime_s(&temptm, &currentTm);

	SYSTEMTIME localTm = { 1900 + temptm.tm_year,

		1 + temptm.tm_mon,
		temptm.tm_wday,
		temptm.tm_mday,
		temptm.tm_hour,
		temptm.tm_min,
		temptm.tm_sec,
		0 };


	TzSpecificLocalTimeToSystemTime(&tmInfo, &localTm, &UtcTm);
	struct tm tempUTC = { UtcTm.wSecond, UtcTm.wMinute, UtcTm.wHour, UtcTm.wDay, UtcTm.wMonth - 1, UtcTm.wYear - 1900, UtcTm.wDayOfWeek, 0, 0 };
	time_t utctm_t = mktime(&tempUTC);

	// Calculate the time difference in seconds
	auto difference = std::difftime(utctm_t, std::mktime(&dbTime));


	// Check if the difference is greater than 30 seconds
	if (difference > 30.0) {
		std::cout << "All fine. Time difference: " << difference << " seconds.\n";
	}

	else {
		std::cout << "Error message. Time difference: " << difference << " seconds.\n";
	}
	return 0;
}

Best regards,

Minxin Yu

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Answer 2

Hi
Markus Freitag,

I think you can use chrono to compare the time obtained from the database with the current time in C++, you can use the <chrono> library for a more modern approach and avoid dealing directly with time_t and struct tm. Here's an example using C++11 or later:

#include <iostream>
#include <chrono>
#include <ctime>

int main() {
    // Database time
    std::tm dbTime = {};
    dbTime.tm_year = std::stoi("2024") - 1900;
    dbTime.tm_mon = std::stoi("01") - 1;
    dbTime.tm_mday = std::stoi("8");
    dbTime.tm_hour = std::stoi("19");
    dbTime.tm_min = std::stoi("05");
    dbTime.tm_sec = std::stoi("44");

    // Current time
    auto currentTime = std::chrono::system_clock::now();
    auto currentTm = std::chrono::system_clock::to_time_t(currentTime);

    // Calculate the time difference in seconds
    auto difference = std::difftime(currentTm, std::mktime(&dbTime));

    // Check if the difference is greater than 30 seconds
    if (difference > 30.0) {
        std::cout << "All fine. Time difference: " << difference << " seconds.\n";
    } else {
        std::cout << "Error message. Time difference: " << difference << " seconds.\n";
    }

    return 0;
}

If this helps kindly accept the answer thanks much.