Until seems to be hardcoded to SINCE + 6 DAYS and giving any value higher than since has no effect to grand_total. If until is smaller than since, API returns error "Date since=2019-05-23 may not be after until=2019-05-22". Otherwise grand_total stays with value SINCE + 6 DAYS.
I have tested it in cURL both in Terminal and PHP and error repeats in both environments.
User error. Used weekly reports api and first line of docs tells that "The weekly report gives aggregated 7 day durations or earnings grouped by users and projects."
Until seems to be hardcoded to SINCE + 6 DAYS and giving any value higher than since has no effect to grand_total. If until is smaller than since, API returns error "Date since=2019-05-23 may not be after until=2019-05-22". Otherwise grand_total stays with value SINCE + 6 DAYS.
I have tested it in cURL both in Terminal and PHP and error repeats in both environments.
EDIT: Additional information for debug
REQUEST: https://toggl.com/reports/api/v2/weekly?user_agent=[user_agent]&workspace_id=[workspace_id]&since=[since]&until=[until]&user_ids=[user_ids]