I have a feeling this may be related to C syntax, but I started my programming life with C++ so I am not sure.
Basically I have seen this:
struct tm
You can see the reference linked below and quoting from there "In C, you must explicitly use the struct keyword to declare a structure. In C++, this is unnecessary once the type has been defined." See the link for more information and examples.
http://msdn.microsoft.com/en-us/library/64973255%28v=vs.80%29.aspx