Where does the software-center store its history?

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http://askubuntu.com – The Ubuntu software center is able to display a detailed history of packages installed /updated/removed according to date. Where does it get this info from? (/var/log/dpkg.log is maintained only for 12 months; I don't know of any apt-* or dpkg commands to get the date when a package was first installed.) How can I copy this data in the form of a table / text to be processed by another program? (HowTos)