It's easier to redefine the p/invoke signature, in this case like this:

[DllImport("kernel32.dll", EntryPoint = "RtlMoveMemory")]
private static extern void CopyMemory([In, Out] byte[] pdst, Mystruct[] psrc, int cb);

define the struct layout for p/invoke:

[StructLayout(LayoutKind.Sequential)]
public struct MyStruct { public float Value; public TimeSpan Value;}

Then you can just use it like this:

CopyMemory(bufferTarget, bufferSource, bufferTarget.Length);