I sowed Sweet Corn last year on 1st April in Richy Newspaper pots. They were soon ready for planting out (couldn't hold them back beyond 1st May to plant out by which time the roots had grown through into adjacent pots and I made a note that 3 weeks from sowing to planting was going to be enough this year.). Note that hardening off needs to start 2 weeks after sowing, maybe even a bit earlier, in order to be completed by 3 weeks from sowing (and hardening off will slow down the growth a bit!)
Planting on 1st May last gave me a struggle to keep the frost off them (with fleece), and the plants were much smaller than my later sowing (sown 6th May, planted out on 2nd June) presumably due to the colder conditions. They tasted OK though
I grew too many plans last year, so we had a glut of cobs, so this year I am sowing 6 every fortnight to spread out the crop - and hoping that 6 planted 3 across the bed will be enough to self-pollinate - once the second lot are planted they will probably help pollinate each other. This year I aim to make my last sowing around 1st June, maybe a bit earlier, as last year's 9th May sowing was all eaten up at the August bank holiday, so a later sowing will get me a bit beyond that.
Last year's 1st April sowing gave me my first cobs on 20th July, and the 9th May sowing was finished by 31st Aug.
This was for Swift F1 which is a short, fast maturing, super-sweet variety - other varieties may have a longer cropping cycle.