I think the keyword is 'offline'.You can buy real time clock chip addons for rpi.
You obviously have a lan though, why not setup your own ntp server? Point all the pis to it. Job done.
My Pies are on their own network without internet access
I tried setting up the ntp on another laptop that is always on but the pies dont update from it
i purchased the RTC chip but after reboots my pi's loose an hour
i created comands b4 in yatse to update library clean library and even skip to the dvd main menu
I was hoping someone could help me to figure out one command that would sync my android phone date and time to my raspberry pi