I suppose there's a typo: **** .
The construction omits the subject **** . The last is a conjunction meaning _but_ , not the subject particle. Literally the phrase means _There is still some time till sunset, but are you gonna leave/set out?_ (The speaker is implicitly assuming they should set out after dark).
* * *
The omission is common for time or distance.
* (e.g. on trains) = it will take a while before we arrive in Tokyo (sounds more like time)
* (e.g. on road) = there is a long way to go before we arrive in Tokyo (sounds more like distance)
* = Let's eat something because there is still some time before dinner.
As seen in these, I think it is more common to use them with some quantification (/), but also works in most cases.