Vegetation phenology, the study of the timing and length of the terrestrial growing season and its connection to climate, is increasingly important in integrated Earth system science. Phenological variability is an excellent barometer of short- and long-term climatic variability, strongly influences surface meteorology, and may influence the carbon cycle. Here, using the 1895–1993 Vegetation/Ecosystem Modelling and Analysis dataset and the Biome-BGC terrestrial ecosystem model, we investigated the relationship between phenological metrics and annual net ecosystem exchange (NEE) of carbon. For the 1167 deciduous broad leaf forest pixels, we found that NEE was extremely weakly related to canopy duration (days from leaf appearance to complete leaf fall). Longer canopy duration, did, however, sequester more carbon if warm season precipitation was above average. Carbon uptake period (number of days with net CO2 uptake from the atmosphere), which integrates the influence of all ecosystem states and processes, was strongly related to NEE. Results from the Harvard Forest eddy-covariance site supported our findings. Such dramatically different results from two definitions of ‘growing season length’ highlight the potential for confusion among the many disciplines engaged in phenological research.