When you click on them, they resolve as March 1-3 - I'm assuming it's due to having 5 weeks instead of 4 in the month. This is on up-to-date Firefox, Windows 7.
Thank you very much for your report, I've been able to reproduce this issue and I've passed it to the developers.
For now, as a workaround, please try to type 3/29 or 3/31 manually into the due date field.
How is March 7, 2013 an invalid date?
Are you kidding me?
You don't need to use the comma - typing "March 7 2013" will work and also the following formats will add the correct due date:
Mar 7 2013
Please refer to this page: https://todoist.com/Help/timeInsert for a complete list of supported due dates.
This issue has been fixed and it's possible to select these dates from the mini calendar.