Question

Since DateTime is a struct with members that appear to break down into simple mathematical values, I’m not sure why using sizeof() on it produces the message in the question title.

Answer 1

Because the CLR can only determine the size at runtime… one of the reasons for this is “padding” (platform dependent)…

For all other types, including structs, the sizeof operator can be
used only in unsafe code blocks. Although you can use the
Marshal.SizeOf method, the value returned by this method is not always
the same as the value returned by sizeof. Marshal.SizeOf returns the
size after the type has been marshaled, whereas sizeof returns the
size as it has been allocated by the common language runtime,
including any padding.

Ref.

see also How do I check the number of bytes consumed by a structure?