Add toolbar button for custom formatting - Excel 2013

In Excel 2013 I love the ability to quickly format a number in the Number section of the Home tab, but one I would love to see is date with time. I don't really care what format, but something like MM/DD/YYYY 24hh:mm:ss would be great. Can I create a custom button on that Number section (unlikely I know) or a custom toolbar button to accomplish this?

Thanks!