No hay mayor diferencia, pero creo que "allá" da una idea de un lugar más lejano que "allí"

¿Qué hora es allá? If I was asking the current time in your country. ¿Qué hora es allí? I don't know if "allí" its right maybe "ahí " its better, sounds natural to me. ¿QUÉ HORA ES AHÍ? its the same as ¿QUE HORA ES ALLÁ? both refer to your country, or the place with different time zone that the person you are talking.

Hi, i don't speak English nothing, nothing.... I from Santiago of Chile or Chili , south America Answer your question, is It means when he speaks of the time in another place ... eg telephone conversation: QUE HORA ES ALLÁIn chile?, ie it is like, QUE HORA ES ALLÍ, refer to the same ... hopefully help you with something I know nothing about any English ... if you know English could be exchanged for you to learn Spanish and I can learn English! :)