Thanks for reaching out to us. I'm here to help provide some insights about start date in QuickBooks Online (QBO).
Are you trying to input a start date when creating Chart of Accounts or Product and Service? If so, QBO will allow you to manually type-in the start date you want, all you have to do is follow the start date format (mm/dd/yy).
If you're referring to something else, please don't hesitate to leave a comment below. Any details you can share that would help me understand the situation would be appreciated.