A stream receives a discharge from an industrial wastewater treatment plant. After mixing in the stream, the ultimate BOD is 14.3 mg/L, the dissolved oxygen concentration is 7.2 mg/L, and the stream temperature is 20 degrees Celsius. At the stream temperature, the saturated dissolved oxygen concentration is 9.2 mg/L. The deoxygenation rate constant (base e) and reaeration rate constant (base e) are 0.2 day^-1 and 0.3 day^-1, respectively. What is the time at which the minimum dissolved oxygen occurs (in days)?