How to Convert a String Representing Time to a `time_t` in C ?

Converting a String Representing Time to time_t in C

When working with time in C , you may encounter scenarios where you have a string that represents a specific time and need to convert it into the portable time_t data type. The time_t type represents time as the number of seconds elapsed since January 1, 1970, 00:00:00 UTC.

Conversion to time_t

In C 11 and later, the std::get_time function can be used to parse a string into a std::tm struct, which can then be converted to time_t using mktime:

#include <iostream>
#include <sstream>
#include <time.h>

int main() {
  std::string time_details = "16:35:12";
  std::tm tm;
  std::istringstream ss(time_details);
  ss >> std::get_time(&tm, "%H:%M:%S");
  std::time_t time = mktime(&tm);

  // Print the converted time_t
  std::cout << "Converted time: " << time << std::endl;

  return 0;
}

Comparing Two Times

Once you have multiple times in the time_t format, you can compare them to determine which is earlier.

#include <iostream>
#include <time.h>

int main() {
  std::string curr_time = "18:35:21"; // Current time
  std::string user_time = "22:45:31"; // User's time
  std::tm curr_tm, user_tm;
  std::istringstream ss;

  // Parse both times
  ss.str(curr_time);
  ss >> std::get_time(&curr_tm, "%H:%M:%S");
  ss.clear();
  ss.str(user_time);
  ss >> std::get_time(&user_tm, "%H:%M:%S");

  // Compare the time_t values
  std::time_t curr_secs = mktime(&curr_tm);
  std::time_t user_secs = mktime(&user_tm);

  if (curr_secs < user_secs) {
    std::cout << "Current time is earlier than user's time" << std::endl;
  } else {
    std::cout << "User's time is earlier than current time" << std::endl;
  }

  return 0;
}

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