I have configured aptitude to write to a log (/var/log/aptitude). It produces output like this;
Aptitude 0.4.11.11: log report
Mon, Feb 9 2009 13:21:28 +0100
IMPORTANT: this log only lists intended actions; actions which fail due to
dpkg problems may not be completed.
Will install 6 packages, and remove 0 packages.
4096B of disk space will be used
===============================================================================
[UPGRADE] apt 0.7.20.1 -> 0.7.20.2
[UPGRADE] apt-utils 0.7.20.1 -> 0.7.20.2
[UPGRADE] base-passwd 3.5.19 -> 3.5.20
[UPGRADE] libgnutls26 2.4.2-5 -> 2.4.2-6
[UPGRADE] libpq5 8.3.5-1 -> 8.3.6-1
[UPGRADE] ucf 3.0015 -> 3.0016
===============================================================================
Log complete.
This shows (obviously :) ) the exact date and packages that aptitude installed. To configure this, follow the aptitude reference;
Option:Aptitude::Log
Default:/var/log/aptitude
Description: If this is set to a nonempty string, aptitude will log the package
installations, removals, and upgrades that it performs. If the value of
Aptitude::Log begins with a pipe character (ie, ``|''), the remainder of its
value is used as the name of a command into which the log will be piped: for
instance, |mail -s 'Aptitude install run' root will cause the log to be emailed
to root. To log to multiple files or commands, you may set this option to a list
of log targets.
You will find a link to the aptitude reference in the aptitude man page.