DO NOT BOTHER
There are those who think that sundials should tell only tell Solar Time. There is a simplicity and historical purity about this approach. The delight in such dials comes their decorative and geometrical design.
But there may also be a degree of political irritation! Santiago de Compostela is 8.5° West, yet its time is based on a meridian of 15° East (Central European Time). Civil midday in the city is more than an hour and a half before solar noon. In Kashgar in Western China, civil noon is at 9 am solar time.
This is perhaps why the beautiful bifilar dial on a beach in Barcelona, below, made in 1996 by Rafel Solar i Gaya, is emboldened with 'Temps Vertader' or 'True Time'.
Interestingly in 2016, The Spanish government was discussing moving its clocks back 1 hour to help the work-family balance in the country - and come closer to True Time.
The alternate approach - that is to say to organise a dial to real standard time - is based on one of 3 possible premises....
... a dial is meant to read time, then it should read the same time that everyone is reading from their watch.
... it's just fun to engineer something that forces nature's time to match mankind's time
... it's sad for a gnomonist to see someone (especially a child) dismiss a sundial as old-fashioned or useless because "it doesn't tell the right time". However, it is acknowledged that only some people will bother to the instructions that usually have to accompany a mean time dial. Thus the education that one is trying to accomplish may be in vain.
It is just a matter of taste whether to correct a sundial or not!