The date that shows up in SELECT @@VERSION is the actual date and time that Microsoft compiled the SQL Server binary. The release date for the CU is when Microsoft released the CU to the public, after they did their testing and documentation.
]]>Microsoft SQL Server 2012 – 11.0.5582.0 (X64)
Feb 27 2015 18:10:15
Copyright (c) Microsoft Corporation
Enterprise Edition (64-bit) on Windows NT 6.3 (Build 9600: ) (Hypervisor)