Parsing date and time string into a number to compare C++

My application receives a date and time string. I need to be able to parse this string and compare it to the current time in seconds.

I am parsing this as below into a struct tm t to get the year, month, day, hour, minute, and second separately.

    std::string timestr = "2020-12-18T16:40:07";
    struct tm t = {0};

    sscanf(timestr.c_str(), "%04d-%02d-%02dT%02d:%02d:%02d",
           &t.tm_year, &t.tm_mon,  &t.tm_mday,
           &t.tm_hour, &t.tm_min, &t.tm_sec);

I'm not sure if I need to convert this to epoch time, but when I do , I get -1. I'm not sure why.

time_t t_of_day;
t_of_day = mktime(&t);

Do I actually need to convert this to epoch first?

What is the best way for me to get the current time in seconds and then compare it to the time information I get in t? Thanks.

You want C++ parsing:

https://en.cppreference.com/w/cpp/io/manip/get_time

std::stringstream timestr = "2020-12-18T16:40:07";
struct tm         t = {0};

timestr >> std::get_time(&t, "%Y-%m-%dT%H:%M:%S");

I should note there is a bug in your code as: tm_year is not the same as year as we know it. This is the number of years since 1900!

https://www.cplusplus.com/reference/ctime/tm/

So your code needs another line:

 t.tm_year -= 1900;

Note: std::get_time() already does that compensation.

This is probably why mktime() is returning -1 as the year 3920 is out of range.