I am not 100% positive on this interpretation, however it seemed to coincide with the time I received this device, so that might be it!
(1) Turn off phone.
(2) Hold VOLUME DOWN while powering on device
(3) Press VOLUME UP / DOWN until it reads "Barcode"
(4) Press Lock button
(5) Read the "Date" row, note it's in MM-DD-YYYY format
To get out:
(6) Press any VOLUME key
(7) Use VOLUME UP / DOWN keys to select "Start"
(8) Press Lock button
I pre-ordered my International Nexus 6 from Google, the device says 12-09-2014 (9th Dec 2014), and I received my phone on the 19th December 2014.
Hope this helps!