E.g. consider the hemming and hawing about a 64-bit time_t. That's a tiny change in comparison, and one that's obviously unavoidable and with a strict deadline. And yet...
I had received a compiler warning about this when trying to compile a program on Raspberry Pi (it was a old version; I did this a few days before they implemented 64-bit time_t on Raspberry Pi). Fortunately I was able to add a macro to allow it to work on computers without 64-bit time_t. Other than that, the program I compiled worked perfectly, although I wrote the program on PC and did not specifically try to make it work with Raspberry Pi. So, a C code is portable, although sometimes a few changes are required to make the program to work.
But I think that 64-bit time_t is a good idea.