Application of Exogenous KH2PO4 and Salicylic Acid and Optimization of the Sowing Date Enhance Rice Yield Under High-Temperature Conditions

High temperature (HT) severely limits rice production, but effective measures can reduce the adverse effects of HT. A 2-year experiment was conducted involving five sowing dates to investigate the effects of potassium dihydrogen phosphate (KH2PO4), salicylic acid (SA), and the sowing date on the physiological characteristics, yield and quality of rice subjected to HT. Under ambient HT conditions (daily average temperature ≥ 30 °C for ≥ 3 days or maximum temperature ≥ 35 °C for ≥ 3 days), KH2PO4 (KP1-KP2: 22.05 and 36.75 mmol L−1), SA (SA1-SA2: 0.5 and 1.5 mmol L−1), and their combinations were applied to leaves from the late-booting stage to the flowering stage, with deionized water as a control (CK). Compared with CK, KP1, KP2, SA1, KP1 + SA1, and KP2 + SA1 alleviated the adverse effects of HT on the number of grains per panicle, seed set, and yield, among which SA1, KP2 + SA1, and KP1 were the best. The antioxidant enzyme (superoxide dismutase, peroxidase, and catalase) activities, levels of osmotic protective substances (soluble sugars, soluble proteins, and proline), and chlorophyll content were increased and the malonaldehyde content and leaf temperature were decreased in the SA1, KP2 + SA1, and KP1 treatments. Compared with those sown on April 25 and May 5, plants sown on May 15, May 25, and June 5 had suitable temperature conditions and presented increased grain yield and quality. Therefore, the application of SA1, KP2 + SA1, and KP1 and the use of an optimal sowing date (from May 15 to June 5) increased the rice yield and quality under HT.