I'm trying to calculate.  What time is the FIDynamic.ArrivalMinutes based off?  Is it off the Unix timestamp?

I'm calculating from the Unix epoch & it's coming off about +/-05 minutes from the ETA showing off your website.

The ArrivalMinutes value is just a simple time computed off of speed and distance to destination. Ultimately, you'll see a little bit of fluctuation in the actual value, especially if the flight changes speed or course during its flight.

After years of trying to find just the right magic formula for computing ETA, we've determined that this is the best/easiest route :)

Ok that's fine.  An ETA off a few minutes isn't a big deal.  I wanted to make sure I was geting as accurate as possible.