This same thing with the date rolling back to May 2014 keeps happening to my brothers 925, although the lockups are far more frequent than yours. Sometimes freezing more than once a day and definitely happening overnight, this has been a problem for him on the developer preview (pre cyan) and official 8.1 too so it is not down to the cyan firmware update. He has no Nokia care centre nearby so he has sent it to me and I've taken it into a care centre for him.
The weird thing is it only does it when the sim is in it, if removed and only using wifi it worked for a week, it never does this with a sim in which kind of defeats the purpose of having a mobile phone, I don't think getting another sim would work either as it has done this on his old sim with Vodafone and after unlocking with his giffgaff sim. I just hope the care centre can figure out what is wrong...
J.
Edit: Just to add, when he could roll back to WP8.0 it works flawlessly and never locks up, with the NSRT he seems to have lost the ability to go back to WP8.0 as the only thing on offer is WP8.1 with CYAN.
Last edited by bridder; 08-13-2014 at 01:49 AM.
Reason: Added WP8.0 details
08-13-2014 01:34 AM