In this work, we used novel time modelling methods in a mostly cognitively unimpaired cohort to demonstrate that plasma p-tau217, measured by Lilly Meso Scale Discovery (MSD) and Alzpath Quanterix, can be modelled to produce individual level estimates of p-tau217 + age and time, plasma p-tau217 positivity is generally an intermediate step between amyloid and tau PET positivity but this timing is highly variable between individuals, and that time from p-tau217 onset is strongly associated with brain tau pathology and cognitive decline. This evidence concerns the gene MAPT and amyloidosis.