Fix https://github.com/OfficeDev/Open-XML-SDK/issues/6 making date W3DTF

core.xml's date elements need to be in precisely the right format for other tools to read them properly.
Specifically other consumers of
https://github.com/OfficeDev/Open-XML-SDK/issues/6

Please add a space before the opening parenthesis (Mono [Coding Guidelines](mono-project.com/Coding_Guidelines)).
This applies to the rest of the changes too.