I too wanted to make a sundial which would be a serious timepiece, i.e. tell the time accurately. I soon discovered that it was not a simple matter. I read about the Equation of Time, Time Zones, Daylight Saving, and the effect of local longitude. However, I set to work with an Excel Spreadsheet and I have come up with a table which gives a single correction figure in minutes for any day of the year, for any longitude on earth, and any local time zone. I can set my watch by my sundial, correct to within 1 - 2 minutes at any time of day.

The table is also useful for establishing the correct alignment of the gnomon when mounting a new sundial. It will give the exact local time at which the gnomon needs to be set to cast its shadow at precisely midday, after which the table will be accurate for any day of the year.

There does not appear to be any way to attach the .xls file to this reply, so if you are interested just email me direct for a copy of the table. I would welcome feedback about it, and suggestions for improvement are invited.
mervmatthews@xtra.co.nz

The xls worksheet sounds like a very good idea. I will email you for a copy as I would love to see it. I've built several sundials, and my best is a prototype equitorial made from a clipboard and cardstock. It is adjustable so it can be set to show watch time. The finished one will be aluminum - I'm waiting for the parts to come from the machinest now.

I have designed a page that allows to create easily paper vertical sundials, You introduce your wall position on google maps and the pages gives to you a printable sundial.

Enjoy: http://sundial.damia.net