Knowledge · Systems

C and C++ Unix timestamps

time() vs clock_gettime() vs gettimeofday()

time(3) returns seconds since the epoch as time_t. It is simple but offers no sub-second data.clock_gettime provides seconds plus nanoseconds for CLOCK_REALTIME (subject to NTP adjustments) or monotonic clocks for measuring intervals. gettimeofday is legacy; POSIX labels it legacy — prefer clock_gettime on new code. On Windows UCRT, use _time64 and friends when targeting 64-bit epochs consistently.

struct tm breakdown

struct tm holds calendar fields. Beware: tm_mon is 0-based while tm_year is years since 1900. Always zero the struct before use or call the *_r variants with an explicit result buffer for thread safety.

strftime format codes (selected)

POSIX vs Windows highlights

MSVC and MinGW differ on which POSIX functions are available without feature macros. When cross-compiling, centralize time access behind a small abstraction and unit-test DST transitions using recorded vectors. Filesystems may store NTFS times in 100-ns intervals — do not confuse those counters with Unix epoch without documented conversion.

Example: epoch to formatted UTC

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

int main(void) {
  time_t now = time(NULL);
  struct tm utc;
  gmtime_r(&now, &utc);
  char buf[64];
  strftime(buf, sizeof buf, "%Y-%m-%dT%H:%M:%SZ", &utc);
  puts(buf);
  return 0;
}

Key takeaways

• Use *_r functions on POSIX for thread-safe field breakdown.
• Monotonic clocks measure duration; wall clocks answer "what time is it?"
• Verify time_t size in firmware builds; assume nothing.
• Prefer clock_gettime over gettimeofday for new code.
• Centralize Windows vs POSIX differences behind one module.

Timestamp converter