Most landlords expect and manage to let properties on one year fixed term agreements (52 weeks). These agreements are binding so you may well find you have to pay for a much longer period than you intend to occupy the property. As there is a surplus of student housing in Newcastle, you may be able to negotiate a more flexible or less expensive arrangement but it will depend largely on the timing of your negotiations, the demand for the house, how you present your case and the individual landlord.
You should not agree to pay from a date which has already passed. You should find out for certain if the tenancy is for a FIXED period e.g. for 12 months from 1/7/2012 ending 30/6/2013 OR if there is no fixed end date (known as a periodic tenancy) where you would be able to give 1 months notice to vacate, although the latter is rare in the student letting market.
If your course is for a shorter period than one academic year (e.g. one semester only), or you expect to be away from Newcastle on placement for part of the year, please seek advice from the Private Housing Team.