Ummm...you can NOT use document.write after a page has loaded. When you do, you wipe out all content on the page except what you just wrote.
And not to ask a silly question, but WHY do you want something so incredibly inaccurate? Nobody uses a Julian calendar any more. What possible real world use could it have?
And the other problem is that using a calendar as you have asked for could easily result in illegal dates. e.g., you could easily end up with FEBRUARY 29, 1900, just to pick the obvious example.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.