Where (in which tiddler) are the DateFormats for the ‘created’ and ‘modified’ fields specified?
I know that I can modify these formats using the $view widget. But where would I do this, i.e. when I want to change their display in the InfoPanel?
Some handling of these key date fields has being automated inside the core and I would advise against changing these, however they are stored in the tiddler in the “TiddlyWiki serial date number format” and can be displayed in anyway you wish. I actually recommend working to store all dates in this format.
Regardless of how the date is stored there are various ways they are or can be displayed in the User Interface, and yes the view widget is common, now there is the possibility in filters as well with the format operator.
I am not sure what you mean by the info panel, are you referring to a particular tab? You can create your own tab by tagging a tiddler with $:/tags/TiddlerInfo and used whatever date format you want there.
I use the link to tabs plugin a lot to find the underlying core tiddler i the User interface tabs of tiddlywiki.
The following might be the relevant tiddlers.
$:/core/ui/TiddlerFields
$:/core/ui/TiddlerFieldTemplate