Format a known timestamp in UTC.

C: gmtime converts to UTC calendar fields and strftime formats them.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <stdint.h>
#include <stdbool.h>
#include <errno.h>
#include <limits.h>
#include <math.h>
#include <time.h>

int main(void) {
    time_t timestamp=0;
    struct tm *utc=gmtime(&timestamp);
    if (utc==NULL) return 1;
    char text[32];
    if (strftime(text,sizeof text,"%Y-%m-%dT%H:%M:%SZ",utc)==0) return 1;
    puts(text);
    return 0;
}

Expected output EXAMPLE

1970-01-01T00:00:00Z

Details

Save as snippet.c; compile with gcc -std=c17 -Wall -Wextra snippet.c -lm -o snippet, then run ./snippet. gmtime converts to UTC calendar fields and strftime formats them.

Keep in mind

This is the Unix epoch on the tested Linux runtime. gmtime may use shared static storage, so use the platform’s thread-safe variant in concurrent code.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:02:40 UTC · gcc (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0 · gcc 4:13.2.0-7ubuntu1

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: gcc -std=c17 -Wall -Wextra -Werror snippet.c -lm -o snippet && ./snippet
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: 80ea60ac5db1f1edef552ddda8fb746cf92458640a65aa71dbdbe4f8bff39c71
Dependencies: gcc 4:13.2.0-7ubuntu1

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

# No input files required.

Observed standard output

1970-01-01T00:00:00Z

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.