The dates for receiving a tax refund after it has been accepted by the IRS are only estimates based on the IRS target of issuing a tax refund after 21 days from the date it was accepted by the IRS. This is a target only and not a guarantee.
After the tax return has been Accepted by the IRS (meaning only that they received the return) it will be in the Processing mode until the tax refund has been Approved and then an Issue Date will be available on the IRS website.
See this IRS website for federal tax refund FAQ's - https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
