Update 2018-11-06: Adobe tells me that the licensing should be good for 99 days, so something is amiss (e.g., a bug of some kind). I have sent the logs from my machine to Adobe for evaluation, using the Adobe Log Collector Tool.
You should be able to work offline for 99 days. I suspect there’s something off here and hopefully the logs and backend details can help us troubleshoot. https://helpx.adobe.com/creative-cloud/kb/internet-connection-creative-cloud-apps.html
Update 2018-11-08: Adobe responded quickly and I had a talk with senior engineer. It seems that the clock on the iMac 5K is losing time and thus time regresses, triggering the Adobe licensing code to disable the license
Adobe’s licensing verification scheme (appropriate word) is problematic if one is away from the internet more than a day or two.
I've been away from home working offline in canyons and such with no internet for an entire month. During that month I've used Photoshop regularly at least a dozen times with internet. In other words, Adobe has been able to verify the same Mac dozens of times in a month. Ditto for every day I’ve been home for the past year. That ought to be good enough for the Adobe verification system!
Making the behavior even more egregious, earlier today I was on the internet for at least 3 hours before coming up again into the high country where there is no internet. Plenty of time for Adobe's background daemon to check with the servers. I might not have launched Photoshop during that time, but it sure would make sense for the ever-present Adobe daemon software to unotrusively do its checks without me having to remember to launch Photoshop.
The more troublesome implication seems to be that I can never plan on being without internet more than some period of time (48 hours?), or it seems that Adobe will shut my work down (at least until this apparent bug is fixed).
Below, Photoshop was working in the mountains at 18:35. Then abruptly around 19:00 it shut off as per the dialog. So the hours of work I had planned this evening (and tomorrow) are kaput. The only way I can make Photoshop work again is an hour-long 30 mile round trip drive 3000 vertical feet down and back up while burning 3 gallons of fuel (about $15 over here, 50% more than the Adobe monthly licensing charge!).