You said your pi is "offline"- which could mean it's just firewalled from the Internet but otherwise connected to at least a local network.
I proxy NTP access to my devices via the router. Let me explain:
My Pi's do NOT get their time set by talking to NTP servers on the Internet- the ROUTER itself is their NTP server. Only the router has external UDP/123 connectivity to external NTP servers to set its' time correctly. In the DHCP config of the router assigning IPs to the Pi's I then specify the router's interface for the Pi's subnet as their NTP server. Below are some illustrative pictures from both the router's side and inside a Pi at end of this post.
Such a config allows the Pi's to keep correct time while ring-fenced from the 'Net. This is also more bandwidth efficient than have a gazillion things querying external NTP servers on the 'Net.
DHCP config Looks like this:
Inside the Pi the file storing our NTP server config is found here:
When we open the file, we see the NTP server is the Router's interface for the Pi's subnet:
So Pi gets correct time without talking on the Internet.