I'd like to track my visitors' time on site of landing page and have tried using Google Analytics but I don't think GA is reliable.
I am trying to filter out traffic sources where visitors don't even spend 5 seconds on the LP.
Is there a specific tracker or do I need a custom script embedded in my LP?
You'll need a custom script for this. AFAIK none of the current generation trackers track this by default: CPVLab used to (sort of) but I don't recommend using that.
However, once you have that custom script I believe both FunnelFlux and Thrive should be able to take in the tracking data including "did they stay for 5 sec" using their Javascript trackers. You could use [this code](https://support.funnelflux.com/14510...ith-Javascript) as a basis on FunnelFlux, and I'm sure the support for both trackers would be happy to help you figure this out.
Not sure if
Alternatively, if you specifically want to check time-on-page against one specific tracking variable, you could just use Javascript dumping the result to another server. It'd be about a dozen lines of Javascript and another dozen lines of code on a server somewhere (or Amazon Lambda) to drop tracking variables and time on page into a database which you could then query separately.