Hi Neil,
for POS data Standard DSiM currently expects weeks starting on Monday.
However, the following workaround exists:
You can use BAdI RSU5_SAPI_BADI to retreive all time intervals available in data source /DDF/TIME_ATTR. Within the BAdI implementation you can adjust the intervals according to your needs and insert the adjusted records to the ones available in /DDF/TIME_ATTR.
Regards,
Patrick