15 years or more. You should be ok unless it's running Windows 98 or ME.
Maybe something like this?? An ntp time server using satellites.
https://www.ebay.com/i/362768894932
I have NO idea how well these work.. maybe also try and find a WWVB to NTP device but I think those are rather more expensive.
Also maybe this for the Raspberry PI:
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You could probably do it with an Arduino, given the very light load.
I have not tried these.. just pointing out the idea isn't new.
https://www.geekstips.com/arduino-time-sync-ntp-server-esp8266-udp/
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Has anyone taken the time to see what a typical clock drift might be? I would guess the processor has some counter that might be off a bit (assuming no 32768 crystal) but I would think it is pretty stable.
I think the bigger issue will be the mobility of the hub. You'll need to keep updating the location to keep sunset/sunrise close to accurate. The clock will drift by seconds, the location may change by time zones.
About 1 year ago I was running a hub without internet and the time would reset to the Date and time the hubs firmware was completed, so it might go back to midnight a month and two days ago during the DB maintenance. It was terrible back then and the NTP driver wasn't available yet so I had to run an AtJob from a windows machine to set the hubs time every hour to make sure it was correct. Once the NTP option was available I changed to using that and shortly after that they supposedly made some changes to try to keep it from changing the date but I never bothered to turn off the NTP driver long enough to see how much it improved.
But doesn't the NTP driver require the internet? Or am I missing something.
It requires an NTP server.. not necessarily an internet connected one.
Arduino + GPS Allows you to make a non-internet NTP server. There are Topics that include this here on the Community.
And of course you can always buy your own cesium clock. 
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Or with an RPi (~$100 all in).
I followed this guide, and it is working great.
Note: a little soldering required on the GPS hat. And the GPS antenna needs to be close to a window to get a good signal (but does not have to be run outside).
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I've seen the RPi used multiple times but it's overkill for NTP... however, NTP & Homebridge & Node-Red & HubConnect Proxy/Server would be cool. 
Which is why I picked Arduino + 'hat'. More fit for One Function. But I do admit buying Arduinio's in a 3 pack and thus have a spare
I also bought a pair of ESP and have one consumed for Hubduino.. so I have a spare ESP.
I also have a Photon I used for the 10 mins to identify that it's cool. So it's spare.
I have a Z-Uno also... I've never powered it up, yet. Many great Ideas for that when I have time.. so that's another spare.
All of them can make magnificent NTP servers
Agreed. Arduino + hat would work very well. I probably should have gone that route, as I have plenty of arduinos and esp8266 in service.
BUT, I had a pile of RPi laying around from another project I bailed on, so just used what I had around (other than the hat).
Seems we share a buying / using principal.
But then I'm not ready to "Move To Wyoming" (aka live without internet) and my Stratum 2 NTP server is working just fine for all my systems. My Cisco ASA is adequate protection for now.
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I really want to go back and put an LCD screen on the RPi and have a time readout, etc. But It is hard to find the time (or a case that can fit a screen AND the GPS HAT).
No time.... no time.... (ironic since we are talking about time servers)
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It just requires a NTP server on the internal network I installed NetTime on a Windows machine and set it to always provide the time even if it hasn't been able to sync for months as that computers clock might drift by seconds over that time period but not days or more like the hub did.
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Does this virtual device exist? I already have a local NTP server on my network and would love the hub to use it. My DHCP server does include the NTP service address in configuration it passes out, but I notice my hub's clock seems to drift so I'm assuming it's not using it.
UPDATE: I found this:
Yup!
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(I was just updating with that link)
Wouldn't you need to have a crossover cable in order to connect to your hub directly from your laptop?
Also, you would have to have something with a DHCP server. You can manually assign an IP address on your laptop but you can't on the hub. The hub has to connect to a DHCP server in order to get an IP address at all.
It would be easier to buy a little travel router with a network connection and use that instead. They are like $15 on Amazon.
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Nothing wrong with the travel router. And a good point about assigning IP, most versions of windows have DHCP services, maybe only pro. You no longer need a CO cable, the laptop NIC will auto negotiate the crossover. Like skinning cats.
