I had to do a bit of search over the forum to find the article which you can use to display the since a member registered, shot out to S!r.ReaDy. for posting it :
You'll have to make a bit of code adjustments to make it display the days, months and years, look over this article for the strtotime : https://stackoverflow.com/questions/2915864/php-how-to-find-the-time-elapsed-since-a-date-time
You can use the if / elseif logic to create the code.
{{if [expression]}}
HTML to output if expression is true
{{elseif [expression]}}
HTML to output if expression is true
{{elseif [expression]}}
HTML to output if other expressions were not true
{{endif}}
For the staff badge, instead of making it for each group in part you can use the "this member is a moderator" badge and customize it.
{{if $comment->author()->modShowBadge()}}
Staff Member
{{endif}}
Remember that this will appear to all the member who the option "show badge" biffed inside the moderator permission.