Tones, you would think, but adding the quotation marks didn't change the result, and it does appear that (single) square brackets are OK, for instance this is fine:
\define testbrackets(one, two) $one$ | $two$
<<testbrackets [UTC]YYYY0MM0DD0hh0mm0ssXXX another>>
(Double square brackets makes the YYYY part bleed into the two parameter, though, because that's one way of quoting a parameter.)
Mark, this was an intriguing possibility, but I opened the tiddler in a text editor and it actually says:
created: NaNNaNNaNNaNNaNNaNNaN
So it does appear to actually be the text that TiddlyWiki saved in the field.
Maybe the only way to answer this one is by diving into the source code to see what special logic happens when the created field is set. Not that it really matters a whole lot. Just my overactive drive to understand exactly why everything doesn't work speaking here. :-)