One or two years seem a little bit too much. If your targeted school requires online submit, you don't have any problem. If you plan to submit paper letter, you may have to get the letter when you apply. It is not good to simply leave the date until you apply. Except the letter, usually your recommender needs to fill out a form and put the form with the letter in an envelop, then seal it and sign across it. He/she has to put the date on the form. So if he leaves the date, he cannot seal the envelop and sign across it now. | 
