A method and charging system for facilitating interim billing for an IMS session is provided. The method may include receiving time-based interim accounting messages at CDFs from NEs (i.e., CTFs) of an IMS network serving an IMS session, opening or updating interim CDRs for each NE serving the IMS session based on the time-based interim accounting messages, extracting accounting record numbers from the time-based interim accounting messages, the accounting record numbers uniquely identifying the time-based interim accounting message in relation to other interim accounting messages from the corresponding NE for the IMS session, and determining if a partial record closing trigger has been reached for the IMS session, wherein the interim processing trigger is a function of the accounting record number extracted from the time-based interim accounting message for the corresponding NE, an AII value, and a partial record closing threshold.