> How did you get this warning?

This looks like runtime output from a program built using Clang/LLVM with -fsanitize=undefined. See here: http://clang.llvm.org/docs/UsersManual.html#controlling-code-generation

Signed integer overflow is undefined behaviour, so by the time *sec = (time_t)intpart has been evaluated, the undefined behaviour has already happened. It is too late to check for it afterwards.