In QGIS 2.4, i am unable to set the default value as NULL if i have not updated any value on date fields. Its automatically taking the current system date. 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. It may be edited if required. Please refer the attached screen shots.
Open up the layer's Attribute Table > Toggle editing mode. Select the fields you want changed then open the Field Calculator. Check the Only update selected features box at the top.
Assuming you already made a column for the Date, check the Update existing field box and select the required field. Then in the expression box, enter
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 recommend that you report it to the QGIS Development team who say:
If you find a bug, please report it!
You need an OSGeo account and login in order to submit bug reports. To get started, first create Create an OSGeo4 account.
Once you have your account, use QGIS issue tracking to search if the issue you'd like to report is probably [sic] already entered.
In the meantime it looks like @joseph has provided you with an acceptable workaround in his answer.
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We are not a GIS Software Support Site. We encourage you to seek official routes for support for specific issues you may have.