C Snippets

C Unix Timestamp Snippets

C examples for getting the current Unix timestamp, converting epoch seconds to a human-readable date with strftime(), and building timestamps from calendar dates using timegm().

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Current Unix timestamp (seconds)

#include <time.h>

time_t ts = time(NULL);

Current timestamp (milliseconds, POSIX)

#include <time.h>

struct timespec ts;
clock_gettime(CLOCK_REALTIME, &ts);
long ms = (long)ts.tv_sec * 1000L + ts.tv_nsec / 1000000L;

Format timestamp as UTC string

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

time_t ts = 1700000000;
struct tm *tm_info = gmtime(&ts);  // UTC
char buf[64];
strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", tm_info);
printf("%s\n", buf);  // "2023-11-15 06:13:20"

Format timestamp as local time string

#include <time.h>

time_t ts = 1700000000;
struct tm *tm_info = localtime(&ts);  // host timezone
char buf[64];
strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", tm_info);

Timestamp from calendar date (UTC, GNU)

#define _GNU_SOURCE
#include <time.h>

struct tm t = {0};
t.tm_year = 2023 - 1900;  // years since 1900
t.tm_mon  = 10;           // 0-indexed; 10 = November
t.tm_mday = 15;
t.tm_hour = 6;
t.tm_min  = 13;
t.tm_sec  = 20;
time_t ts = timegm(&t);   // GNU/POSIX; use mktime() for local

Time difference in seconds

#include <time.h>

time_t start = time(NULL);
/* ... work ... */
time_t end = time(NULL);
double elapsed = difftime(end, start);

These C examples use Unix seconds for the portable baseline and show milliseconds only where the runtime exposes them naturally. The question to answer before copying a snippet is simple: what unit does the next API expect?

Get the current Unix time

#include <time.h>

time_t now = time(NULL);

Convert a timestamp to UTC

time_t seconds = 1700000000;
struct tm *utc = gmtime(&seconds);

Production note

time_t is the conventional Unix-time type, but its width and range depend on the platform. Use the UTC conversion functions when the output must not depend on the host timezone.

Keep the unit in the field name when a value crosses a boundary: createdAtSeconds and createdAtMs are longer than createdAt, but much less mysterious during an incident.

Epoch to Date Date to Epoch Timezone Converter All Code Snippets

Frequent questions:

Q: What type should I use for Unix timestamps in C?: A: Use time_t for second precision and int64_t (from <stdint.h>) when you need to store milliseconds or nanoseconds. Avoid int or long because their sizes are platform-dependent.
Q: How do I get milliseconds in C?: A: Use clock_gettime(CLOCK_REALTIME, &ts) from <time.h>. The result is in ts.tv_sec (seconds) and ts.tv_nsec (nanoseconds). Compute milliseconds as ts.tv_sec * 1000LL + ts.tv_nsec / 1000000.

C Unix Timestamp Snippets

#Get the current Unix time

#Convert a timestamp to UTC

#Production note

Frequent questions:

Get the current Unix time

Convert a timestamp to UTC

Production note