json - Client get new data from server -

i designing web app server generates batches of data, , client periodically checks whether new batches of data available download. way doing whenever server generates new batch of data, available @ particular url. client periodically checks url see whether new batch available it. (i not using web sockets.) batch of data in format of json object.

since have little web experience, i'm bit confused when client visits url. how should client know whether batches of data @ url new (in case client should download them) or old (in case client should ignore them, since has downloaded them in past)?

also, there may multiple clients working same server, solution should work regardless of number of clients.

include timestamp property (through server side script) in json thrown server. need change value of timestamp property everytime update data server. easy detect change checking modification date.