Steps to reproduce:

1. PHP 8.3.6, Phorge at 61c953268ecd72e5dce6faeae042922eb7ef8fbd
2. Enable the Facts prototype application at http://phorge.localhost/applications/view/PhabricatorFactApplication/
3. Go to a project's Reports page (like http://phorge.localhost/project/reports/1/ ) of a project which has several tasks associated and also saw changes in the number of associated tasks over time
4. In the Burndown chart, hover over a data point and look at the date string in the tooltip

Actual outcome:

YYYY-M-D

Expected outcome:

YYYY-MM-DD