ip route expiry time goes negative

This seems to only be a display problem, though I don't know if it's the
kernel or iproute2 that needs attention here...

$ ip -6 route add 2001:789::/64 dev eth1 expires 5
$ watch ip -6 route show dev eth1
Every 2.0s: ip -6 route show dev enp0s9  myhost: Tue Jul  2 10:07:33 2024

2001:2:0:1000::/64 proto ra metric 2048 pref medium
2001:789::/64 metric 1024 expires -15sec pref medium <<< negative expiry!
fe80::/64 proto kernel metric 256 pref medium

Notice the -15sec expiration time.

The route really does become unavailable when it reaches zero, so this
seems to be a display problem only, as can be demonstrated with:

$ watch ip -6 route get 2001:789::1

After 5s, the route becomes UNREACHABLE.

Matt.