I didn't remove this functionality, the Date column has always been dynamic in that it shows text to fit the column width, so if you make the column smaller you get an abbreviated date, wider and you get it fully spelled out with day name, month name etc. Probably Yosemite's new system font doesn't fit so well with your previously set column widths. So, just like in the Finder, set the column wider - hover the mouse over the right edge of the column header, click and drag to resize
