Date issue since WordPress 3.5

We store the last login date in a custom field as a unix time stamp. It used to display correct in WordPress 3.4.2 utilising the “date” column, but it now displays dates as either today, or the same date as today in thousands of years time.

We upgraded the plugin this morning to the latest version but the problem remains.

https://www.ads-software.com/extend/plugins/codepress-admin-columns/