How to make sense of update.txt tile

[supunkamburugamuve]

I'm trying to benchmark Storm and cannot figure out how to calculate the latency. The document says to calculate latency with the formula

window.final_event_latency = (window.last_updated_at – window.timestamp) – window.duration

I believe (window.last_updated_at – window.timestamp) this is the value in update.txt file. For window.duration of 10 seconds, I get values below 10 seconds in the update.txt file essentially making latency negative.