The chill accumulation model is used to estimate the number of days from harvest (or leaf fall) in the previous season until bud break in the current season. It is well known that tree crops and natural tree species have specific chill requirements, which are related to the number of accumulated hours within a known temperature range. In this model, we use chill days (C_D), which are defined as the cumulative number of hours below a pre-selected threshold temperature divided by 24 hours per day (to express as accumulated days rather than hours). We also use "anti-Chill days" (C_A), which are defined as the cumulative number of hours above the same pre-selected threshold temperature divided by 24 hours per day. The C_D values are given a negative sign and the C_A values are given a positive sign. In the model, the C_D values are accumulated until they reach a pre-selected value that is identified as the chill requirement (C_R). Because the C_D values are negative, the C_R is also negative. The chill requirement is met on the day when the SC_D is less or equal to C_R. On the following day, the model no longer adds C_D, but begins to add daily anti-chill days to C_R. On the date that C_R + SC_A is equal or greater than 0, that date is selected for bud break. The optimal value for the chill temperature threshold and for C_R are determined using trial and error until the root mean square error between predicted and observed days between harvest or leaf drop and bud break is minimized.