Thermal time (TT) is an agro-climate index widely established and used in predicting plant development based on temperature. This index is a powerful tool for measuring multi-faceted changes in temperature occurring from climate change. In the present study, TT was calculated for the entire frost-free period and individual spring, summer, and fall seasons using growing degree day (GDD), general thermal index (GTI), crop heat unit (CHU), and heat stress degree day (HSDD) models for 1054 counties across 12 Midwest states on a daily basis from 1950 to 2017. The temporal trend for each county was fit with a linear regression model for percent change per year. During the frost-free period, warming occurred in 260 to 489 counties with 0.06 to 0.34% gain per year dependent on model and county selected. Warming has occurred in northern and eastern counties primarily from gains in the fall season and partially from the spring. These TT gains are from additional calendar days from an expanded frost-free period and secondarily from a change in maximum temperature (fall only). Heat stress (>30°C) during the frost-free period has decreased for 212 counties in the west-central region. Overall, the CHU model detected the most counties warming and had the lowest error particularly compared to the GDD model. Compared to 1950, some counties showed up to 1.2-fold increase in frost-free TT and are projected to 1.8-fold by end of the 21st century. Current warming trends are related to projected TT trends such that adaptation planning can be guided by the trajectory from the past 68 yr.