AlhpaBeta, thanks for the (as always) thoughtful suggestions.
Prefixing the macros does reduce the likelihood of name collision but it makes them less Arduino like.
I could change them to inline functions so the compiler will warn if the user provides a function with a similar name. Lets see if there other opinions on how best to address this.
The name tmElements_t was chosen because this structure was really intended for programmers writing time provider code rather than use in a sketch (I wanted to keep the tm prefix to remind programmers that its derived from the C tm structure). But I can certainly change the name if it looks like TimeElements could be useful in a sketch.