-Start an xterm.
-Find out which files are included in an application: dpkg --list name-of-package
-When you know the names of the files (or you knew one in advance), pick one of the non-mutable ones (e.g. one in /usr/bin or /bin or /sbin, for example), and list the date of last change: e.g. 'ls -lc /usr/bin/maemo-mapper' (without the quotes)
- The OS installation date is more tricky, because it's been installed by flashing a full image (and this will also make the above trick for individual applications fail for applications pre-installed with the OS image). However, the following _might_ just work: ls -ld /etc/rc0.d