Planting date is a commonly manipulated management tool in soybean [Glycine max L. (Merr.)] production, and earlier planting has been shown to increase soybean yield and influence seed quality. A majority of U.S. growers now plant soybean before 15 May, weeks ahead of historical planting dates. The effects of agronomic improvements, including earlier planting, on soybean yield gain and seed quality have not been quantified. The objectives of this study were to determine the effects of earlier soybean planting on (i) soybean yield gain, (ii) soybean seed quality, and (iii) seed mass over time. Research was conducted in 2010 and 2011 at Waseca, MN, Arlington, WI, Urbana, IL, and West Lafayette, IN. 59 MGII cultivars (released 1928-2008) and 57 MGIII cultivars (released 1923-2007) were planted at target dates of 1 May (early) and 1 June (late), representing a distribution of historical release years. A mixed-effect regression analysis was used to model change over time in yield, seed quality, and seed mass parameters for each maturity group. Preliminary results indicated a 20% greater rate of yield improvement (p<0.018) in MGIII soybean cultivars for early May vs. early June plantings, suggesting earlier planting has played an important role in soybean yield gain. No significant differences in the rates of yield gain were present among MGII soybean cultivars at the two planting dates. Earlier planting affected yield, protein, oil and seed mass (MGII), as well protein content (MGIII); it did not impact seed mass for MGIII. Cultivar release year affected yield and oil content (MGII), as well as protein, oil and seed mass (MGIII); no impact was noted on protein and seed mass (MGII).