F965 Why is the Sunrise/Sunset Glance off by about 5 minutes from any other source

A few minutes difference for me too across FR 965, various apps and websites however I'm not going to obsess over a few minutes here or there - it's hardly a life or death matter.

One minute difference in sunset vs distance. 
The Earth rotates 360 degrees in approximately 24 hours, which means it rotates about 15 degrees per hour, or 0.25 degrees per minute. At the equator, the Earth's circumference is approximately 40,075 kilometers, so each degree of longitude corresponds to about 40,075 km/360=111.32 km. Therefore, a one-minute difference in time corresponds to about $0.25\times 111.32=27.83\text{km}$ at the equator. Obviously it will be a shorter distance the further north or south of the equator you are. 

I've noticed the same, and it's difficult to know for sure why the difference is what it is. One possibility I've thought about is that maybe (just maybe) Garmin calculates the times based on the middle of the sun, whereas most other sources give sunset as the time when the top of the sun disappears behind the horizon (and similarly sunrise as the time when the top of the sun appears over the horizon). That might explain the couple of minutes difference.

(Then there's the atmospheric refraction that causes the sun to become visible about 2 minutes before the "real" sunset/sunrise time. And the fact that the time when the sun becomes visible depends also on your altitude and the altitude of the horizon. So I definitely agree with RichardB, there's no point in fretting over a couple of minutes, it's not going to be exact anyway.)

RichardB  Agree, definitely not a life or death difference. I'm just curious.  Clearly there's a difference in methodology, because Garmin is consistently different than the other sites mentioned.  Bitti Interesting.  It would be nice if someone from Garmin could give some insight.  Bug or difference in methodology?  I'm guessing the latter. 

Yes, this is curious!

I checked the sunrise and sunset times at my location for today. Garmin says 7:46 / 17:34. Heavens-above.com, which I trust for astronomical accuracy, says 7:44 / 17:36. So indeed two minutes of difference! And Garmin thinks that the day is shorter :-)

I think I can explain why. Heavens-above.com determines the sunset / sunrise times when the Sun is 0.8 degrees below the ideal horizon. Sun's angular diameter is about 0.5 degrees, so mathematically this is well below the horizon. But as explained already by others, we see the Sun before the mathematical sunrise and after the mathematical sunset due to atmospheric refraction. I believe Heavens-above defines the sunrise / sunset times for when the tip of the sun becomes visible or disappears. Two minutes is typically the time "added" by refraction, so I suppose Garmin determines the sunrise / sunset times when the Sun is 0.25 degrees below the horizon, i.e., the mathematical sunset / sunrise. I suppose you'll just see the complete Sun above the horizon at the moments reported by Garmin, assuming an ideal horizon like at the sea.

In practice you should probably care more about twilight. And there are three well defined ones: civil, nautical and astronomical. I wish Garmin computed all of those and made it configurable which times to show. And maybe show all visually on the day/night glance bar? That would be a true outdoor feature :-)

I never thought about it in those terms. Generally, living in "sunny" Scotland seeing the sunrise and sunset is weather-dependant and it's a rare occasion when the weather gods smile on us!

According to Wikipedia the time of actual sunset is defined in astronomy as two minutes before the upper limb of the Sun disappears below the horizon. Astronomically, sunrise occurs for only an instant: the moment at which the upper limb of the Sun appears tangent to the horizon. However, the term sunrise commonly refers to periods of time both before and after this point:

This Wikipedia article (on sunset) is inconsistent with another Wikipedia article (on atmospheric refraction) which says:

"By convention, sunrise and sunset refer to times at which the Sun's upper limb appears on or disappears from the horizon and the standard value for the Sun's true altitude is −50′: −34′ for the refraction and −16′ for the Sun's semi-diameter."

Perhaps we should look at a more reliable source, like https://www.ga.gov.au/scientific-topics/astronomical/astronomical-definitions

"Sunset is defined as the instant in the evening under ideal meteorological conditions, with standard refraction of the Sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon."

In any case Garmin does not seem to take atmospheric refraction into account and is off from both the astronomical model and reality :-(

Appreciate the reply, but I'd prefer a more international source for the definition of sunrise and sunset.  But your site seems to agreen with the US Navy and the NY Times. :-)