Most compilers would give you a "file not found" type of error if an include file is not found. Whether or not the file is in sys or not depends upon the compiler. Have you checked for the presence of the file?
Header files do not include the actual functions. They merely declare their form. The library that contains the actual code must be present and be able to be found during the linker phase. This is unlikely to be the problem with something like time, though.
You should always give information about your system, OS, and compiler; particularly for stranger behavior regarding system library code. Perhaps you could also show a snippet of the code where the error occurs, in case of syntax errors at or shortly before the point where the error is flagged.