A calendar heap structure and method for sorting N timestamp (TS) values in an ATM fabric scheduler implemented in an access network element. The heap structure is implemented using a radix value (R) for grouping the N TS values into a number of groups. Validity bits associated with the TS values are hierarchically arranged in a tree having log.sub.R(N) levels, wherein the bottom layer's N bits correspond to pointers that point to linked lists of flows with respective TS values. Starting from the top level heap, each subsequent level's heaps are successively examined for determining which particular heap obtains a minimum value until a particular validity bit of the bottom level is reached, which validity bit points to the minimum TS.