If the Mahr is not paid at the time of the Nikah, is it necessary to stipulate a future date for payment?

Answer

Yes, she can demand it. It is her right. Since she can demand cash payment, she has the right to defer payment to a specified future date. If on the due date the husband is unable to pay, the wife will surely grant him time to pay. The need for fixing a date is necessary to obviate a recalcitrant husband's refusal to pay. He could argue that he will pay it after fifty years, and never end up paying.