How do I update a custom date in C++? - c++

Question

I created my own DateTime class. It accepts the current date/time, and it also accepts a custom date/time. The custom date/time is what I'm interested in.
If I set the date to 1/5/1953 with a time of 1:05:31 PM, and call updateTime(), I want the time to update based on the difference between when it was first created and how many milliseconds followed afterwards.
However, when I do this, it's always giving me today's date and time, which is not the desired result.
This is my current code.
if (m_isCustomDate)
{
time_t currentRawTime;
// Get the current raw time
time(&currentRawTime);
// Get the time lapse
time_t time_diff = (time_t)difftime(currentRawTime, m_rawTime);
// Increment the time difference to the old raw time
m_rawTime += time_diff;
// Update the tm structure
localtime_s(&m_tm, &m_rawTime);
}
Updated problem:
With a date of date to 1/5/1953 and with a time of 1:05:10 PM, and when I call getSecond(), it's not giving me the 10 as I expect, but it gives me the current second on my computer (4 in this case). Is localtime_s() not the right function to use in this case?
My getSecond() function:
/// <summary>
/// Gets the current second between 0 and 60.
/// </summary>
/// <returns>Returns the second.</returns>
int DateTime::getSecond()
{
updateTime();
return m_tm.tm_sec;
}
Updated specific question:
How can I get the time lapse between the original custom date (m_rawTime) and the time lapse since the app's start up, and then update the tm structure?
Edit:
This solution worked. Posting if anyone needs a working example:
const DWORD curr_time = GetTickCount();
DWORD time_diff = (curr_time - m_init_time) / 1000;
m_rawTime += time_diff;
localtime_s(&m_tm, &m_rawTime);
m_init_time = curr_time;

Answer 1

Given that m_rawTime holds the 'custom' time, I noticed the following. With time_t time_diff = (time_t)difftime(currentRawTime, m_rawTime); you get the difference between the current time and m_rawTime. Then, with m_rawTime += time_diff; you make m_rawTime equal to the current time. I think, this is not what you want to do.
You say you want the time to "update based on the difference between when it was first created and how many milliseconds followed afterwards". So, you effectively want the difference between the time the object was last updated and the current time. To do that, initialize some counter (say, this->init_time) to the current time in the constructor and make each call to updateTime add the difference between the current time and this->init_time and make the latter equal to that current time:
void DateTime::updateTime() {
const auto curr_time = get_time(); // this is not an actual function
const auto diff = curr_time - this->init_time;
m_rawTime += diff;
// update the tm structure here...
this->init_time = curr_time;
}
Now, if you want to work with milliseconds, microseconds or smaller time periods, you should use std::chrono::high_resolution_clock, but struct tm doesn't support time periods shorter than one second, so you can't actually make your custom time any more precise than that with it. In other words, if you stick to struct tm, you can only work with precision of one second, no more.