For Liferea v1.1.8 the date format handling was changed. All previous versions had a preference that allowed you to choose between two default date formats and a user-specified date format. To satisfy the usability gurus out there this preference was dropped.
The new default format is an item age-dependant format as you can see in the screenshot. So items will have descriptive date labels with relative or day of week names. Items older than a week will additionally have the name of the month. Items not from this year will additionally contain the year and will have the exact time dropped.
This date formatting might be familiar to you. The simple reason being that it was copied from the Evolution source. The reasoning is when you have no user defined date formatting in Evolution why should Liferea need it?
For the few user who really did use the user-specified date format preference: there is a new GConf preference "/apps/liferea/date-format" where you can specify your favourite format string. Note: the old user-specified preference was saved into "/apps/liferea/timeformat", so you might just want to copy the content.