The reason is that the built-in types like int are platform-dependent. So on one computer an int might be good enough to store a time value, while on another you would need a long. To allow people to write programs that run on all platforms, types like time_t were introduces, which are often just alias definitions for some primitive type that is suitable on that particular platform. This in fact takes a little more learning effort in the beginning, but this effort will pay large dividends in the long run.
Makes sense, doesn't it?
[EDIT]: As for the strange warnings: The compiler is warning that the conversion of a time_t and suseconds_t to double might lose some information. That is because both types are integer types with more bits than the mantissa part of double. In your case that would apply only to very large time values, so you can simply ignore these warnings. (But how should the compiler know that a time_t value normally fits into a double? So he emits this warning.) There is in fact not much you can do about that without making the code platform-dependent.
