Unless you tell TurboTax otherwise, TurboTax is correct to use the default treatment dictated by the tax code that assumes that your income was received uniformly throughout the year. To do otherwise it's your responsibility to allocate your income to the various tax quarters on Schedule AI of Form 2210. You'll do this as VolvoGirl describes, not during the entry of the Form 1099-R. It's impractical to specify the dates of income receipt during the entry of various items of income because this would have to be done for every type of income. Also, each form reporting income can represent multiple income events.
