How to subtract metric from time value

I want to see the current pod age. Since there is no metric like that, but only the pod creation time (got that from kube_pod_created{namespace="$namespace"} * 1000), I though that maybe I could subtract that from the current time of the host machine. Is that possible? If so, how can I do that?

yep, there is a way. See the time() function in PromQL.

Also check out this panel in one of our public dashboards and then try something like:

time() - node_boot_time_seconds{job="node", instance=~"$node:.*"}

or I guess in your case something like:

time() - kube_pod_created{namespace="$namespace"} * 1000)