Activation times (AT) describe the sequence of car- diac depolarization and represent one of the most impor- tant parameters for analysis of cardiac electrical activ- ity. However, estimation of ATs can be challenging due to multiple sources of noise such as fractionation or base- line wander. If ATs are estimated from signals recon- structed using electrocardiographic imaging (ECGI), ad- ditional problems can arise from over-smoothing or due to ambiguities in the inverse problem. Often, resulting AT maps show falsely homogeneous regions or artificial lines of block. As ATs are not only important clinically, but are also commonly used for evaluation of ECGI methods, it is important to understand where these errors come from. We present results from a community effort to compare methods for AT estimation on a common dataset of simu- lated ventricular pacings. ECGI reconstructions were per- formed using three different surface source models: trans- membrane voltages, epi-endo potentials and pericardial potentials, all using 2nd-order Tikhonov and 6 different regularization parameters. ATs were then estimated by the community participants and compared to the ground truth. While the pacing site had the largest effect on AT cor- relation coefficients (CC larger for lateral than for septal pacings), there were also differences between methods and source models that were poorly reflected in CCs. Results indicate that artificial lines of block are most severe for purely temporal methods. Compared to the other source models, ATs estimated from transmembrane voltages are more precise and less prone to artifacts.