Degree-day (DD) estimates based on different numbers of daily temperature readings recorded at regular intervals were analyzed for precision and bias. Given a temperature threshold below the daily minimum, one median standard deviation was 0.067 + 0.103, −0.043 (95% confidence limits) DD. Bias in the degree-day estimates was detected as a function of time of year, and the relative position of the threshold between the minimum and maximum temperatures, however, the bias was less than 0.030 + 0.00193 (SEM) DD per day. Hourly temperature data provided a better estimate of degree-days than mathematical functions fit to minimum and maximum temperature data. Use of mathematical functions results in function specific bias that changes with time of year. In those instances where the timing of phenological events has been based on a particular function, continued use of the function is necessary for consistent results.