Last Frost Dates for Belvidere and Rockford

I have had many new gardeners ask "When is the last frost date for the Belvidere/Rockford area?" I usually plant with the date of May 15 as being safe. Belvidere and Rockford are in an area that gets different temps than Chicago. I consider us a zone 4 although many sites and charts list us as zone 5. I plant as if we are zone 4. After doing a lot of research on the internet, going to official government sites, the Farmers Almanac and extension sites, I will say that most of them had April 28th as the last frost date. The Farmers Almanac had at a 50% probability date; around May 15. I am planning on putting some lettuce seeds in the ground right around April 30 just to see what happens. Otherwise I'll wait till May 15th. Better safe than sorry.