Apple doesn’t allow third party apps to change their icons dynamically.
You can ask Apple to add this possibility by leaving them feedback here:
http://www.apple.com/feedback/iphone.html
As an alternative Week Calendar is able to show either:
- Day of Month,
- Day of Year
- Week Number
- Remaining Events Today
- Today’s Events
Please see https://weekcalendar.zendesk.com/entries/23274216 for further reference
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