Contract Invoice Start End Dates

This module connects contract management with invoice date tracking. When a contract line is turned into an invoice line, the module checks whether the product on that line is flagged as needing start and end dates.

If the product requires dates, the module calculates the billing period from the last date already invoiced up to the next recurring date, and copies that period onto the invoice line as its start date and end date.

• No new menus or screens are added; the change happens silently when invoices are created from contracts.
• The behaviour only applies to products marked as "must have dates"; other products are invoiced exactly as before.
• It touches the contract line (contract.line) and the invoice line it prepares, so the dates appear on the generated invoice lines.

In practice, users no longer have to type the covered period by hand on invoices coming from contracts: the period is derived from the contract's own schedule. This keeps invoice lines consistent with the subscription or service period they bill.