Time and formatting in C

time gives seconds, localtime breaks them apart, strftime formats them.

#include <stdio.h>
#include <time.h>

void stamp(void) {
    time_t now = time(NULL);
    struct tm *local = localtime(&now);

    char buffer[64];
    strftime(buffer, sizeof buffer, "%Y-%m-%d %H:%M:%S", local);
    printf("%s\n", buffer);

    printf("day %d of year %d\n", local->tm_yday + 1,
           local->tm_year + 1900);
}

double seconds_since(time_t past) {
    return difftime(time(NULL), past);
}

How it works

1. time_t counts seconds since the epoch.
2. localtime returns a pointer to shared static storage.
3. strftime is where the format string lives.

Keywords and builtins used here

• NULL
• char
• double
• return
• seconds_since
• sizeof
• stamp
• struct
• time_t
• tm
• void

The run, in numbers

Lines

18

Characters to type

371

Tokens

103

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 62 seconds.

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Step 1 of 2 in Time, step 5 of 14 in Systems programming.

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