An pretty accurate way to retrieve this information, which isn’t specific to a distro is to use the passwd command with the -S option.

This option displays the account status information, including the date of the last password change.

It is safe to assume that these users never changed their password, hence, that date is the installation date.

~
❯ sudo passwd -S sys
sys L 04/20/2016 0 99999 7 -1

~
❯ sudo passwd -S daemon
daemon L 04/20/2016 0 99999 7 -1

~
❯ sudo passwd -S bin
bin L 04/20/2016 0 99999 7 -1

Credits to Alex Ballas