Planting Time Confusion

I have been doing some research to try to figure out when I should actually plant my seeds and seedlings. According to the local Cooperative Extension, the last frost date is March 15. The manager at the local farmer's exchange says the last frost date has moved and is now around April 15. So, do I go with the first date, the second date or split the difference?

Seed shelf

Seed shelf
Originally uploaded by Carolina Victory Gardener

I asked my gardening companion to help me put together a plant shelf that would hold my seed trays, allow for the plant lights to move up and down and, most important, protect the same from our mischievous year old lab. His design is a little chunky, but the seedlings are safe!

Addendum to Planting List 2009

Seed Packets
Originally uploaded by Carolina Victory Gardener

The seeds I already listed were from the Victory Garden Collection. Here is the rest of my list of vegetables, flowers and herbs along with a few other possibilities.

Onion-Parade
Carrot-Yaya
Lettuce-Salad Bowl Mix
Radish-Cherry Belle
Spinach-Melody Hybrid
Swiss Chard-Bright Lights
Borage-Borago officinalis
Basil-Genovese
Basil-Purple
Basil-Sweet
Basil-Lemon/Lime Mix
Oregano-Mediterranean
Parsley-Krauser
Gomphrena-QIS Carmine
Nasturtium-Burpee's Border Mix
Calendula-Keblona Mix
Cosmos-Bright Lights Mix
Aster-Red Burst
Marigold-Boy O Boy mix
Marigold-Chameleon

I'd also like to add some potatoes, monarda, tansy and maybe a cantalope. I haven't decided on the particular varieties of these. Any suggestions?