I would try to judge sowing dates and transplanting dates by the date of your average last spring FROST, not your USDA hardiness zone. (Or, for eggplants, your average last cold night.)
The hardiness zone only reflects the coldest temperature your area is likely to see in an average year.
One good way to judge the best dates for your area is to find a nursery with experienced growers and a lot more concern for their customers than the big box stores. When THEY start selling a certain kind of seedling, THAT is a good time for your region. Home Depot and supermarkets will set out seedlings whenever optimists and procrastinators are likely to buy them!
We do have a "Northeast" regional forum, but they have gardeners from Maine to Connecticut.
Consider Zone 8 which you can find in parts of the country as different as Texas and coastal WA. In the PNW, it gets warm enough to harden off and plant out tomato seedlings around the same time that some parts of Texas are getting temps hot enough to stop their FIRST tomato crop from setting fruit.
We have mid-winter nights of similar coldness, but WA has cold and cool spring nights MUCH later than Texas.
My "average date of last spring frost" is April 6 or April 19, depending on which weather station is more similar to my microclimate.
Dave's planting calendar takes that into account, so the question remains "why does planting advice differ?"
Some advice will be optimistic and list a date for the earliest that your plants will probably not be harmed much by cold as seedlings. They might also think that the "slow maturing" varieties taste better or yield better or just that you might have slow maturing varieties and a short growing period, so they "go early".
Another source might pick the safest date much later, sure to be warm enough for seedlings and just assuming that you have a long growing season and/or fast-maturing varieties.
Watch for advice on a seed packet like "and re-sow every 2-3 weeks for continuous supply". That means that you could sow on the stated date OR much later and still get a crop.
I would say that when you are new to a region or new to growing a certain crop, start one small patch early, one in the middle and one later. One or two of those dates should grow obviously better than the other(s) and next year you'll know when to start that variety in your yard.
This is why crop labels outside need the sowing date (and transplant date) as well as the variety.
I never keep good records, but that is the only way to remember what worked well and what you learned NOT to do again!
http://garden.org/ideas/view/R...
Or, if there's only room for one crop, be cautious and start it at the later date suggested if your springs are prone to late cold snaps. Or be cautious and start it early if it is a cool-weather crop and you're concerned that your scorching summers will dry it out before it matures.
Providence summers won't fry eggplants on the vine, will they? So you only need to worry about an early Fall cold snap shortening your growing season.
Eggplant is so heat-needy and New England springs are so whimsical and untrustworthy that I would "go late" and wait for weather to be WAY past frost, very settled with warm days and not-too-cool nights before setting out eggplants. Just be sure to pick a variety with DTM short enough that they will mature during the steady-warm season.
(If you were growing cool-weather lettuce and spinach, the shoe would pinch in the other direction, and you might "go early", hoping for a mild spring, because you know that "cool weather" will be over by late May or early June.)
