I think a more obscure character like “^” makes sense too. I used “!” because the most popular comment/suggestion uses it, and I was looking for a way to implement the idea. The calendar popup would only trigger when the character is preceded by a space, just like the current tag selector popup works. So exclamations! shouldn’t be affected.

Also worth noting, clicking on an existing date tag would trigger the calendar popup (for editing) and would not filter like the current # and @ tags. Maybe the popup, has a “filter” or “search” button too.

Regarding the “predefined” date formats: To have truly usable dates, WF will have to convert dates (under the hood) into UTC (Coordinated Universal Time), which makes supporting a bunch of user defined date formats (or tag hybrids) difficult.