Aside from @Brucanna’s very valid point, what you say, @dgray, reflects an apparent misunderstanding of the purpose of the Start Date. The Start Date is the date at which a transition is made from a previous accounting system to Manager for an ongoing business. A startup company should not set a Start Date at all. All transactions prior to the Start Date are ignored with the exception of sales invoices and purchase invoices. This is so opening balances in Accounts receivable and Accounts payable can be established by entering invoices not paid as of the transition date. This is necessary so the subsequent payments @Brucanna mentioned can be allocated to specific invoices. Payments and Receipts entered for dates prior to the Start Date are ignored, because as far as the program is concerned, they never happened. See these Guides:
https://forum.manager.io/t/setting-up-starting-balances-for-customers/5494
https://forum.manager.io/t/setting-up-starting-balances-for-suppliers/7227