To avoid breaking backward compatibility what you typically do is allocate new system call numbers for the system calls that have the new bigger time_t. Then at

To avoid breaking backward compatibility what you typically do is allocate new system call numbers for the system calls that have the new bigger time_t. Then at some point you spin a new version of the C library (and in the case of time_t, probably lots of other libraries as well). That allows old binaries to run unchanged, as long as you retire them before 2038. The tricky part starts if you also have to keep the old libraries updated with security patches.