JSON has several advantages over XML. Its a lot smaller and less bloated, so you will be passing much less data over the network – which in the case of a mobile device will make a considerable difference.
Its also easier to use in javascript code as you can simply pass the data packet directly into a javascript array without any parsing, extracting and converting, so it is much less CPU intensive too.
To code with it, instead of an XML library, you will want a JSON library. Dates are handled as you would with XML – encode them to a standard, then let the library recognise them. (eg here’s a library with a sample with dates in it)
Here’s a primer.