1. Create a date field in a database activity.
2. Add an entry, select a date from the date picker. Save.
3. Click the 'Edit' cog to edit this entry, the data picker changes to the next day!
It is a bug.
Because, a user who just want to edit an entry for other field, not mean to change to next day.
If they don't check carefully, they won't notice the changes secretly sneakily made by Moodle.