You can found in time.c in Linux kernel this comment:

 * WARNING: this function will overflow on 2106-02-07 06:28:16 on
 * machines were long is 32-bit! (However, as time_t is signed, we
 * will already get problems at other places on 2038-01-19 03:14:08)
 */
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This means that depending on your platform the bug can happen earlier enough to begin to think about it now.
Nevertheless this depends on the size of your time_t type that is usually based in the long c type(in linux I mean). So since kernel 2.3 the developers are thinking about extending this to 64 bits to avoid this kind of problem. I don't know about other platforms, but I think it is easy enough to handle this problem, and this will be addressed soon.

Zenn