It seems to me that it has always been the date of the first installation. Now that you point it out, I think it would be interesting to have both. 1. Installed - 2. Last updated the - two useful information I think. In this regard, perhaps it would be nice to have an option to display the current version for members somewhere, so that they say to themselves, oh there is something new on the site. What do you think ?
I am pretty sure it is related to the initial installation date of UNA itself. I do think however, the date should reflect the update or both. It would give better information as to when an update that was installed and could have possibly caused a system crash or problem with the script. This would allow better recovery of site and data backups.