Two things to check:
Folder Actions
1. Press Ctrl+Alt+* and choose Tree | Actions from the pop-up menu. Zoot will display all folders with actions attached.
2. Hover your mouse over the icon of each folder in the Folder Tree to see a description of the action. If any works on the Due Date field, that might be the culprit.
Due Date Field
1. In the Item Grid, right-click on the Due Date column header and choose Edit Field from the pop-up menu. Zoot will display the Field Properties dialog.
2. See if there are any Start Delimiters that might be in play.