how to get the date of the last upgrade?

[Meike Stone <meike.stone@googlemail.com>]

Hello,

how can I get information when the last upgrade ("update") was done
(apt-get upgrade or aptitude *-upgrade).

I have a few machines here, and I want to know when the last "system
update" was executed.

The Distributions are debian 5 and 6

Kind regards and thanx for help

Meike