I'm no expert, but at least STM32's default is newlib or newlib-nano, which it inherits from the ARM GCC toolchain[1] from what I gather. I just checked and here it's defined as the following by default:

    #define _TIME_T_ __int_least64_t   
    typedef _TIME_T_ time_t;

[1]: https://imagecraft.com/blog/2019/04/embedded-gcc-libraries-n...