`service_start_date` (line-level service period start)
service_start_date (line-level service period start)
service_start_date records when the service period begins for this invoice row. Bookkeeping uses row-level service periods when one invoice contains multiple service windows, because header-level dates are not precise enough for correct periodization.
Row-level service dates keep period allocation reviewable without reopening invoice documents.
Example values: 2026-02-01, 2026-01-01.