query to fetch tables after a certain timestamp not working - php

Question

I am trying to calculate the total earnings after a certain timestamp.
What I want to do:
I have a button which on click adds the total earnings till that time to the table with a timestamp.
As soon as it gets added to the database, from there on i want to display the total earnings earned after that timestamp.
So, that means if there are no earnings after the click of the button, it should show me 0.
Problems:
The thing works only the first time if the database is empty. Otherwise Values are correctly inserted in the database but the jquery doesnt update the span tags each time I hit the button.
HTML:
<span id="wb_uid8">Earned: </span><span id="wb_uid9">Fetching data..</span>
<input type="submit" id="Button1" name="Button1" value="I received my incentive today!">
<span id="wb_uid10"><strong>Last payment receive date:</strong></span><span id="wb_uid11"> </span><span id="wb_uid12"></span><span id="wb_uid13"><br>
</span><span id="wb_uid14"><strong>Received amount:</strong></span><span id="wb_uid15"> </span><span id="wb_uid16"></span>
jQuery:
<script>
$(document).ready(function()
{
$('#Button1').click(function(){
var fullDate = new Date();
var twoDigitMonth = fullDate.getMonth()+"";
if(twoDigitMonth.length==1){
twoDigitMonth="0" +twoDigitMonth;
}
var twoDigitDate = fullDate.getDate()+"";
if(twoDigitDate.length==1){
twoDigitDate="0" +twoDigitDate;
}
var currentdate = twoDigitDate+"/"+twoDigitMonth+"/"+fullDate.getFullYear();
var dataString = 'date='+currentdate;
$.ajax({
type:"POST",
data: dataString,
url: "receipts.php",
cache:false
});
return false;
});
});
</script>
<script type="text/javascript">
$(function(){
setInterval(function(){
$.ajax({
url: 'checkearnings.php',
success: function(data){
var data=JSON.parse(data);
$('#wb_uid9').html('₹ '+ data['earned']);
$('#wb_uid12').text(data['lastdate']);
$('#wb_uid16').html('₹ '+ data['lastamt']);
}
});
}, 1000);
});
</script>
checkearnings.php
<?php
include 'connect.php';
$earned = 0;
$lastdate = "";
$lastamt = "";
$timestamp = "";
$sql = "SELECT * FROM receipts ORDER BY date DESC LIMIT 1";
$result1 = mysqli_query($db, $sql);
if($details = mysqli_fetch_array($result1)){
$lastdate = $details['date'];
$lastamt = $details['amt'];
$timestamp = $details['timestamp'];
}
else{
$lastdate = "Never";
$lastamt = "Nothing";
}
$sql = "SELECT amt FROM sales WHERE timestamp>'$timestamp'";
$result = mysqli_query($db, $sql);
while($fetch_options=mysqli_fetch_array($result)){
$earned = $earned + $fetch_options['amt'];
}
$responses=array('earned'=>$earned, 'lastdate'=>$lastdate, 'lastamt'=>$lastamt);
$return=$responses;
//echo $earned;
echo json_encode($return);
?>
receipts.php
<?php
include 'connect.php';
include 'checkearnings.php';
if (isset($_POST['date']))
{
$newdate = $_POST['date'];
$time = round(microtime(true)*1000);
$sql = "INSERT `receipts` (`date`, `amt`, `timestamp`) VALUES ('$newdate', '$earned', '$time')";
$results = mysqli_query($db, $sql);
mysqli_close($db);
}
?>