Just guessing......Is it a DHCP connection? In DHCP, the IP is "leased" for a set period and then renewed.
Here, the lease time is settable in my router. The external connection is also DHCP, but I do not know how to check those settings (I suspect the ISP would not want me to....
At the client end, I wonder if there is some record of how long it has had the DHCP lease.
Search Google using "DHCP"---learning more about this than I know should take maybe 5 minutes--tops