Enter 2011 in the date field. Click save or enter other field
-> Date information is gone

Enter Date in the bibtex code as Date = {2011} , the correct year is shown in the Date-Field
Try to modify it, e.g. to 2012.
-> Date information is gone
In some cases the date information is deleted from the bibtex code as well

@tobiasdiez Sounds to me as if there is something wrong with the databinding