Is it a requirement to only run this on newly imaged Macs? Or could you target any systems that don't have the specified icons in the user's Dock? Its not necessarily easier to do it that way, but I was just wondering.
There isn't anything built in to the Casper Suite that can get you the date a Mac was enrolled with your JSS. It may be included in the upcoming version 9, but not sure.
Luckily you do have a couple of options. Here are just two. I'm sure there are several others:
1 - Create a script that runs during imaging or after reboot that will create a new file tucked away somewhere on the system with the date as its contents (using the 'date' command in a shell script)
Couple the above with an Extension attribute that would read back that file. This will obviously only help going forward, not for anything already imaged and in a user's hands, so this may not be ideal.
2 - Use an Extension Attribute to pick up the creation date on something you know was created brand new at the time of imaging. This isn't actually as simple as it sounds. Its relatively easy to pull creation date in a script, but most files on the system will have creation dates that are older than the image date unless you are literally installing a new OS with each image run, and even then most files and folders will have an older date. Something to think of targeting would be one of the package receipts that get dropped into /Library/Application Support/JAMF/Receipts/ Maybe even a receipt for one of the apps you mentioned.
To get the creation date you can use mdls, which is a command line spotlight tool for reading back metadata.
mdls -name kMDItemFSCreationDate /path/to/file/or/folder
Its even possible the /Library/Application Support/JAMF/Receipts/ directory would give you the enrollment date, but that depends on how your imaging is done and if policies run on the Macs right after enrollment, etc.
mdls -name kMDItemFSCreationDate /Library/Application Support/JAMF/Receipts/ | awk '{print $3,$4}'
That outputs something like 2013-07-12 15:31:10
If the results from the folder itself don't look right, then dig a little deeper and target one of the receipts in that folder instead. There may be other files and folders that would give you reliable results too.
Good luck.