I have often worried about references to the Equation of Time in publications. Authors write things like “Sundials are only right on 4 days in the year”, ignoring the fact that the EoT quoted everywhere is for use on the Standard Meridian only - Greenwich here in the UK.
I don’t live at Greenwich, in fact I live quite close to 3 degrees West - 12 minutes 8 seconds behind Greenwich time to be precise. So, where I live, there are only 2 days in the year that clocks and sundials coincide - I refuse to get drawn into the argument about which is right. If you live more than about 4 degrees either side of your standard meridian, sundials and clocks never read the same time.
Some bespoke dial makers put an EoT on their dials, without incorporating the longitude correction - in my opinion that is missing a good opportunity.
To add the longitude correction to the EoT correction every time is a pain, so I devised the EoT slide rule shown. The scale of minutes in the centre is moveable. You slide the double arrow on the scale until it is opposite to the place where you are on the map, and read the total correction, EoT plus longitude, off the date scale.
In the picture, the rule is set for my location, near to Liverpool. You can, of course, substitute an East West scale in place of a map of to make it universal, or add a map of whatever country you like.
Makes things a lot easier.