Tried the following code on Linux / 64 bits.

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

    int main(){
        printf("%d\n",sizeof(time_t));
    }


    $ gcc -o time time.c
    $ ./time
    $ 8

Therefore time_t is 8 bytes at least on 64 bit systems, even when you are using default time_t and not time64_t.

No idea when it was changed. 20 years back?