The documentation, master.ec, and biller.ec document the need to specify MM DD YY as parameters, and specifically pass MM YY to write_billing_summary. This command, however, will interpret a YY as the two-digit year, e.g. "16", and consider this invalid because it is less than "89" (1989). It works to use 4-digit years, but the doc and exec_coms should be updated to reflect the need to specify a 4-digit year.
Closing as ticket moved here: http://multics-trac.swenson.org/ticket/38