Working with calendar dates and clock times in C++ often requires bridging the gap between raw timestamps and human-readable formats. The standard library provides std::time_t for representing absolute moments as the number of seconds elapsed since the Unix Epoch (1970-01-01 00:00:00 UTC), and std::tm for breaking down those moments into calendar components.

The std::tm structure stores individual date and time fields. Developers must account for its specific indexing conventions: months are zero-indexed (0 for January through 11 for December), the year field counts years since 1900, and the day of the week and day of the year follow zero-based indexing as well.

When performing arithmetic on calendar dates, direct modification of std::tm fields can result in values that exceed their valid ranges. Passing such a structure to std::mktime automatically normalizes these out-of-bounds values, rolling over hours into days, months into years, and handling leap years or daylight saving time adjustments.

The following implementation demonstrates how to capture a current timestamp, apply an offset, normalize the result, and render it into a formatted string:

#include <iostream>
#include <iomanip>
#include <ctime>
#include <sstream>
#include <chrono>
#include <cstring>

std::string renderCalendarTime(std::time_t epochVal) {
    std::tm* brokenDown = std::localtime(&epochVal);
    std::ostringstream buffer;
    buffer << std::put_time(brokenDown, "%Y-%m-%d %H:%M:%S");
    return buffer.str();
}

int main() {
    auto wallClockNow = std::chrono::system_clock::now();
    std::time_t rawSeconds = std::chrono::system_clock::to_time_t(wallClockNow);

    std::tm dateFields;
    std::memcpy(&dateFields, std::localtime(&rawSeconds), sizeof(std::tm));

    dateFields.tm_hour += 24;

    std::time_t normalizedSeconds = std::mktime(&dateFields);

    std::cout << "Adjusted Output: " << renderCalendarTime(normalizedSeconds) << '\n';
    return 0;
}

The std::localtime call converts the raw epoch seconds into a calendar structure. By copying the structure into a local variable (dateFields), modifications can be performed safely without relying on the static buffer returned by the standard function. Adding 24 to the hour field intentionally pushes the value out of its standard [0, 23] range. Invoking std::mktime immediately triggers a recalculaiton that propagates the overflow into higher-order fields (days and months). Once normalized, the updated epoch value is passed to the formatting helper, which utilizse std::put_time to produce the final string representation.