I'm in Lancaster County (zone 6b) and I start my tomato plants mid March. If I start them earlier, they get tall and spindly. They actually should be short and bushy when planted outdoors. You should be ok starting yours mid March, too.

I've seen those frost dates for zone 5 and I don't understand it either. Unless they mean the last HARD frost date, not the last light frost date. Our last frost date is around April 20, but local farmers recommend not planting warm-weather vegetables before May 15. They just sit there anyway until the soil warms up.