Vague docs: return value of pn_selectable_get_deadline() undefined

In the docs for the pn_selectable_get_deadline() function the return value is undefined. Is this milliseconds? Nanoseconds? Is this a relative time from now, or an absolute time since 1 Jan 1970?

https://qpid.apache.org/releases/qpid-proton-0.10/proton/c/api/group__selectable.html#ga8a9c9a60982e79b7e648b84fdd0443f9