How do you add time together?

Question

I'm trying to convert time from UTC time/date to what time it would be in Christchurch New Zealand. PS, I am a beginner coder.

I can easily identify the timezone offset and take into consideration daylight savings. My question is, how can I add UTC and the offset time (+1200)

int timeConverted (int utcTime, int timezoneOffset) {

    int answer = 0;
    int totalTime = utctime + timezoneOffset;

    int timeAdded = totalTime;

    if (timeAdded > 2359) {
        answer = timeAdded - 2400;
    }

    return answer;

When I do this I get some funny looking answers. This is for Adelaide, Australia time

Input is 1659 Out is 189 (the answer should be 229)

I can only use if statements (no arrays or anything like that)

Answer 1

If I understand your question, and you want to take the UTC time and timezone offset and then compute local time (for Adelaide, Australia with +930 timezone offset), then you have 3 cases to handle:

1. are the resulting minutes greater than 59? (if so add 40)
2. is the resulting answer greater than 2359? (if so subtract 2400)
3. is the resulting answer less than 0 (if so add 2400)

You can handle that in a fairly simple manner:

int timeConverted (int utcTime, int timezoneOffset) {

    int answer = utcTime + timezoneOffset,
        utcmin = utcTime % 100,         /* consider minutes */
        offmin = timezoneOffset % 100;  /* from each time */

    if (utcmin + offmin > 59)   /* handle minutes 60-99 */
        answer += 40;

    if (answer > 2359)          /* answer greater than 2359 */
        answer -= 2400;

    if (answer < 0)             /* answer less than 0 */
        answer += 2400;

    return answer;
}

Putting it altogether in a simple example that takes utcTime as the first argument to the program (1659 default) and takes timezoneOffset as the second argument (1200 default), you could do something like the following:

#include <stdio.h>
#include <stdlib.h>

int timeConverted (int utcTime, int timezoneOffset) {

    int answer = utcTime + timezoneOffset,
        utcmin = utcTime % 100,         /* consider minutes */
        offmin = timezoneOffset % 100;  /* from each time */

    if (utcmin + offmin > 59)   /* handle minutes 60-99 */
        answer += 40;

    if (answer > 2359)          /* answer greater than 2359 */
        answer -= 2400;

    if (answer < 0)             /* answer less than 0 */
        answer += 2400;

    return answer;
}

int main (int argc, char **argv) {

    int utc = argc > 1 ? strtol (argv[1], NULL, 0) : 1659,
        tzo = argc > 2 ? strtol (argv[2], NULL, 0) : 1200;

    printf ("utcTime : %d\noffset  : %d\nlocal   : %d\n",
            utc, tzo, timeConverted (utc, tzo));
}

(note: to set timezoneOffset both arguments are required)

Example Use/Output

$ ./bin/tzoffset 1659 930
utcTime : 1659
offset  : 930
local   : 229

Look things over and let me know if you have further questions. (also let me know if I misunderstood your question)

Yes, now 2359 845 works correctly, e.g.

Example Use/Output

$ ./bin/tzoffset 2359 845
utcTime : 2359
offset  : 845
local   : 844