AIX was late to the game for sure, but I don't think it was quite that late.
It was more like AIX 4.3, that added the ability to run 64-bit code on a 32-bit OS. As part of that effort all the syscalls were double defined, one for older 32-bit only applications and another for 64-bit applications running on the 32-bit OS. By AIX 5, there was a native 64-bit kernel as well. Pretty sure that between those events the header files were tweaked so all newly built applications were using the 64-bit time_t calls unless a compatibility flag was defined.
It was 5.3 that gained the 64-bit time API. Prior to that, there was only 32-bit time_t, even on 64-bit AIX. See Redbook SG247463 section 5.19 ("Date APIs past 2038").
It was more like AIX 4.3, that added the ability to run 64-bit code on a 32-bit OS. As part of that effort all the syscalls were double defined, one for older 32-bit only applications and another for 64-bit applications running on the 32-bit OS. By AIX 5, there was a native 64-bit kernel as well. Pretty sure that between those events the header files were tweaked so all newly built applications were using the 64-bit time_t calls unless a compatibility flag was defined.