There is a problem with the week number calculation in datefields at the end of a year. When setting the format to Y/W and e.g. select December 30th, 2014, it wrongly shows 2014/01 instead of 2015/01. It seems to me that it is correctly decided, if the week covering two years at the same time (if any) belongs yet to the old or the new year, but the year calculation is broken. This problem has been around for a while. I believe that I've already seen it sometime in Ext JS 4.x.

Steps to reproduce:

* load fiddle http://ift.tt/1up2eM6

* select one day out of December 29th to 31st, 2014

* realize that it shows 2014/01 and that this text is wrong

* select one day out of January 1st to 4th, 2015

* realize that it shows 2015/01 and that this one is correct