As a result of testing QTBUG-26847, it was found that QDateEdit will also temporarily display a short or long day name as its numerical value under the same circumstances as the aforementioned bug.
- QDateEdit has the display format set to 'yyyy/MM/ddd' (or any format with a short or long day name)
- The date value is edited to '31' (cycle through the days by holding the up key)
- The focus is changed to month with Shift + Tab
- The month value is edited to '2'
- The date value shows '29' until cursor is moved or focus changed (or 291 if https://codereview.qt-project.org/#change,33204 is not applied)
The reason this happens is because QDateTimeParser doesn't distinguish between day names and numbers:
Attached is a patch (to go on top of the aforementioned change) which contains unit tests demonstrating the issue.