By law, you must have a reasonable basis for stating that a product can be shipped within a certain time. If your advertising doesn't clearly and prominently state the shipment period, you must have a reasonable basis for believing that you can ship within 30 days.

If you can't ship within the promised time (or within 30 days if you made no promise), you must notify the customer of the delay, provide a revised shipment date and explain his right to cancel and get a full and prompt refund.