The preposition а means direction. It answers the questions
¿A dónde? or ¿Dónde? (=Where? In which direction?):
a Madrid = to Madrid
a la Universidad = to the University
When we are going to be at somebody, we say a casa de:
a casa de Javier = to Javier´s house
With hours (la una, las dos, las tres,...) a
indicates a certain moment, and should be translated as at. It answers
the question ¿A qué hora? (= What time?):
– ¿A qué hora es el concierto? – What
time does the concert start?
– A las dos y media – At half past two.