This investigation was carried out at South Valley Univ., Exp. Farm, Qena, Egypt for estimating the nature of the genetic system governing days to heading by involving parents in a 7 × 7 diallel cross and their F₂-generations, which were planted under two different sowing dates in 2022/2023 and 2023/2024. The mean squares for environments, genotypes, and genotypes × environments interaction were highly significant for all studied traits. Late planting date decreased all studied traits as compared with normal planting date. At normal and late planting dates, the response to selection for earliness as percentage of deviation from the F₂-populations varied from -17.30 (P₄ × P₆) to -3.81% (P₄ × P₅) and from -12.12 (P₅ × P₇) to -1.52% (P₄ × P₅), respectively. Additionally, at normal and late date, the correlated response in spike length, 100-grain weight and grain yield was reduced; the first and second traits were more affected than third. As expected, both additive and dominance types of gene actions control the inheritance of this trait; however, the additive type of gene action was most important in the inheritance of earliness. Both additive and non-additive genetic variances were highly significant in the F₂-generations under both conditions in both seasons. Furthermore, days to heading had high narrow sense heritability under heat stress conditions in both seasons, but a low one under favorable conditions. Consequently, direct selection for earliness at heat stress is expected to be more effective than indirect selection.