Diabetic hyperglycaemia promotes the production of advanced glycation end-products (AGEs), which play a significant role in the development of complications associated with type 2 diabetes mellitus. Vaccinium angustifolium, a medicinal plant used for the treatment of diabetes, produces a variety of phenolic metabolites with putative anti-diabetic activities. To assess optimal cultivation time, seasonal changes in the concentration of six phenolic compounds in leaves and twelve compounds in stems were examined using HPLC-DAD and examined in relation to seasonal changes in AGE inhibition activity, assessed with a fluorescence-based assay. A seasonal decline occurred in the concentration of chlorogenic acid, rutin, and quercetin 3-arabinoside in leaves and chlorogenic acid in stems. The concentration of (+)-catechin, and (-)-epicatechin in stems declined within two weeks before rising and fluctuating insignificantly. AGE inhibition activity of leaves was significantly greater at the final compared to the initial collection date whereas the activity of stems did not change significantly. Relative to the leaf extract, the stem was a more potent inhibitor of AGE formation, which could be a result of the unique phytochemistry of stems. Together, these results revealed significant seasonal variation in the phenolic profile and anti-glycation effects of V. angustifolium extracts and indicated late summer as the collection time yielding optimal activity.