Here are some averages.

This is from naturalgardening.com - It doesn't exactly give Chester County info, but you can get a good estimate.

Last Frost / First frost
Pennsylvania Altoona May 6 October 4
Harrisburg April 9 October 30
Philadelphia May 15 October 15
Pittsburgh April 20 October 23
Scranton April 24 October 14
Williamsport May 3 October 13

growquest.com

Pennsylvania Altoona May 6 October 4
Harrisburg April 9 October 30
Philadelphia March 30 November 17
Pittsburgh April 20 October 23
Scranton April 24 October 14
Williamsport May 3 October 13

For Delaware County, I am going by 4/14 and 10/14. But, these are averages. You still need to be careful with frost tender plants. Cloches, cold frames, row covers, etc. can help protect from light frosts.

Thanks! That helps!
I thought the extension office would have a link or something on it with the dates (but nooo, as my 12yo would say).