Solution
= Solution
The transformation $t=(x-\overline x)/s_x$ centers day at zero and measures it in standard-deviation units. It reduces numerical scale differences and makes the intercept-like parameter refer to the middle of the observed time range. In <nonlinear regression> this can reduce posterior parameter dependence and improve <Markov chain Monte Carlo> mixing. It also makes priors on the growth coefficient easier to interpret; it does not change the curve family.