What is the best time for sowing real Geraniums (not Pelargoniums).?
You can sow geranium seed in early spring. They will need a little heat, 15C is recommended - so it would be good to do it in a greenhouse or on a warm windowsill. I have known geraniums grown from seed to take a long time to grow into large plants so you will probably need to be patient. However that will depend on the species / cultivar that you have seed from. Good luck and let us know how it goes.
I've been reading up on geraniums prior to planting some seeds myself. They frequently vary quite a lot from their parents if they are home (or park) collected. I know this from experience too, since my attempts at growing purple leaved varieties produced some green, some purple, one black and one with perlagonium-like rings of red on it's leaves. More fun than getting them pre-packed if you ask me This year I am going to try and grow seed from a scented variety that was sitting next to the purple ones. You never know.
Germinating seems to be quite easy, and I agree about the sowing time - early spring worked for me.