It is working correctly, the feed has the timezone specified as T00:00:00Z which means Greenwich Meantime. In mojoPortal we adjust it to local time for display. If your local timezone has a negative offset from GMT like -1 hours or more, then it will correctly adjust to the previous day.
So really the problem boils down to the fact that they aren't really using a time in their datetime, so it is just 12am, ie the beginning of the day in GMT and with the correct adjustment for local time it shows as the previous day. It is a true fact that at 12am in Greewich on August 22, it is still August 21 in timezones with a negative GMT offset.
You could probably solve this using some external service to modify the feed before it is consumed in feed manager. Yahoo Pipes for example provides tools for manipulating feeds and producing a new feed with your manipulations. You could just add 12 hours to the date to make it 12pm instead of 12am and that should solve it for your time zone since 12pm GMT would still have the same date when adjusted to local time.
Hope it helps,