Hi Pete,
Try to get the previous month from the sy-datum field:
pre_mnth = sy_datum+4(2) - 1.
form the prevoius month date as :
concatenate sy-datum(4) pre_mnth '01' into pre_mnth_date.
Then use the FM 'RP_LAST_DAY_OF_MONTHS' to get the last day of the previous month.
You can use the date found from the function module in your custom report.
Hope it helps.
Regards,
Munesh.