How to test

Do not set a production clock to 2038. Build a disposable image, feed it the boundary values, and watch logs, files and certificates.

Values worth using

Unix seconds	UTC	Why
2147483646	2038-01-19 03:14:06	Last-but-one good second
2147483647	2038-01-19 03:14:07	Last representable signed 32-bit second
2147483648	overflow	Does the API accept it, reject it, or wrap?
now + 10 years	varies	Backwards Risk for a 10-year warranty

C smoke test

#include <stdio.h>
#include <time.h>
int main(void) {
  time_t t = (time_t)2147483647;
  printf("time_t bytes=%zu last=%s", sizeof(time_t), ctime(&t));
  t = (time_t)2147483648U;
  printf("plus1=%s", ctime(&t));
}

If sizeof(time_t) is 4, you are looking at a time32 ABI. If it is 8, the language binding is clear; your files and columns may not be.

Operational tests

Create a file, set its mtime past 2038, unmount, remount, stat it.
Insert a row with a 2039 timestamp; read it back.
Issue a certificate that expires in 2040; see whether the verifier accepts the dates.
On NTP clients, check documented era handling before February 2036 — a different bug.