Comparing subdirectory names

I am trying to reformat data from one private directory and reformat it and move it to a public one, but i only want to get directories that have not already been moved to the public directory. Here's what i'm working with

Dir1 contains folders for each named with timestamp

20090320081302
20090327080738
20090410081604
....

Dir2 contains folders named as such

20090320-Topic1
20090327-Topic2
20090410-Topic3

what i want to do is compare the first 8 characters of the folders in dir1 and the first 8 char of folders in dir2 and get only the ones that are not in dir2 and (then go thru all of them and rename them accordingly and upload to dir2 but that's another post i suppose i think i can do that easily)

Any help would be great with comments as i'm new to unix scripting.

what have you tried till now??

I was trying to work off the info in this post but not sure what it's doing exactly some comments just on that would be helpful, going to do more research tonight.

http://www.unix.com/shell-programming-scripting/64341-compare-file-names-different-directories.html