Can I use a formula in an Explore dashboard text field?

All I've got is a janky workaround, and you're probably not going to like it. If you don't want to spend a long time on this, make your dashboard time filter highly visible so that people can see the dates that are filtered. Way easier.

But if you're foolhardy like me, you can trick the system into doing what you want it to do.

1. Make a new pie chart query
2. Set the time constraints you want (or do it in the dashboard)
3. Put the pie chart's legend on the left
4. Make the legend really, really wide
5. Set the legend colour for every single possible item to your dashboard's background colour (if you can).
6. Set the legend text to exactly the size of the text you want to display in your dashboard.
7. Embed the query in your dashboard. If next to other text, turn off grid snapping to position it exactly where you need it.
8. If there is something to the left of the text (such as more text), make sure your new query is placed below it, or you'll get a white circle covering your other text.

There definitely should be functionality like you're talking about, but in the meantime, this is a workable (if slightly frustrating) short-term solution. It was easier for my use case because I only needed months (so I only had to set 12 legend items to white), but if you don't want to set 365 days that way, you could also try hiding it behind something.

If you're looking for a particular date format, you can make a custom attribute for it - I may be able to help you with this.