Look at the data that the Arduino receives. It contains an HTTP "GET" request. The contents of the GET request tells the Arduino what data is being requested. It's all spelled out in the HTTP protocol specification.
Just what I said. There are several sites that deal with reading serial data. One is:
Reading client data is done EXACTLY the same way.
Once you have read, and stored, all the data, THEN you can make use of what the client requested.
Go back and look at the questions I posed earlier. Suppose that you have structured your code to generate responses, expecting the questions in that order. You have your code looking for "date" as the trigger for providing a response. If I ask the 4th question first, your code won't answer the first three questions, when I ask them after I ask how "old" you are.
If you read and store the whole question, you can look for "date" in the question, and "time" and "system" and "old" and provide the right response, regardless of the order that I ask the questions in. If you look for "date" in the stream of questions, and I ask for the date last, I will get no answers for the other three questions.