When going to Widgets -> Preview of Widget (choosing arbitrary widget) iframe and flash URLs do have static date.
Idea for improvement:
For example buttons where user can choose whether she wants to use today or yesterday date (and URLs will change accordingly, not to yesterdays date but something like 'date=tomorrow-1'.
today-1, doh. My mistake.
For yesterday, use: date=yesterday
Might be useful (in this context) if the calendar allowed for date or period selection.