Planting date is a commonly manipulated management practice in soybean [Glycine max (L.) Merr.] production; however, the impacts of past and ongoing agronomic improvements, such as earlier planting, on genetic yield improvement and associated changes in seed protein and oil have not been evaluated. The objectives of this study were to determine if a 30-d difference in planting date affected measured rates of genetic improvement in (i) yield, (ii) seed mass, and (iii) seed protein and oil in the midwestern United States. Research was conducted at Arlington, WI, Urbana, IL, and Lafayette, IN, during 2010 and 2011, using 59 Maturity Group (MG) II cultivars (released 1928-2008) at Wisconsin, and 57 MG III cultivars (released 1923-2007) at Illinois and Indiana, with targeted planting dates of 1 May and 1 June. Earlier planting provided higher yields (+3.1 kg ha-1 yr-1) than late planting in MG III soybean. Seed protein concentration decreased linearly over cultivar year of release at a rate of 0.191 (± 0.069) g kg-1 yr-1 for MG II, and 0.242 (± 0.063) g kg-1 yr-1 for MG III. Seed oil concentration increased over year of release at a rate of 0.142 (± 0.037) g kg-1 yr-1 for MG II, and 0.127 (± 0.039) g kg-1 yr-1 for MG III. The interaction between planting date and cultivar year of release for MG III yield suggested that the trend toward earlier planting is one agronomic improvement that, when coupled with genetic improvement, has provided a synergistic increase in on-farm soybean yields in the midwestern United States.