My reading of your question suggests that you are wanting to report what you perceive to be a bug in QGIS 2.4.I have no plans to test that personally, but from reading what you have presented it looks to me like it could well be one. I am expecting to set the default values(NULL) for date fields in properties of layer, since that default values should be displayed while capturing the new feature itself. I still think what that what I suggested (opening Field Calculator and creating or updating Date columns) would do that for you instead of editing the widget attributes.
I can re-edit my post with more images for a step-by-step guide if you prefer?I know about Field calculator method but what i'm saying is in QGIS 2.4 when we capture the new features its automatically showing the current date by default and also we are unable to remove that date to NULL values manually, since i may update any valid dates on some features and i may leave the field without updating any date value on some features.Insightful, hilarious, and a brilliant summary of every article I’ve ever written.In QGIS 2.4, i am unable to set the default value as NULL if i have not updated any value on date fields. I am expecting to set the default values(NULL) for date fields in properties of layer, since that default values should be displayed while capturing the new feature itself. All fields would then have NULL as their date and you could update any fields manually as you mentioned.
On this situation we are unable to know which features should be changed to NULL by using Field calculator method.So that i'm expecting to set default values as NULL for all features if i have not updated any dates but its not as of now.