The Customer Start Date field represents the date the Customer became a Customer. If you create an estimate or an opportunity for this customer, this field will be updated with the date of that transaction. Over time the Start Date field may not reflect the true start date of the Customer.
As a work around, create a new custom date field to store the customers original start date. The field can be manually set. Or you could also create a saved search that finds the customers original start date and populate this field.
1. From a Customer record, click Financial.
2. Start Date - This is the system generated Start Date field.
3. Original Start Date - This is a custom field that was created to record the original start date.