You would use Query & QueryTask similar to what's shown in this example. The example uses the location where you click on the map but in your case, you would create a point using your lat/long values and then pass that as the value used with query.Geometry.
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no"/>
<title>Simple Map</title>
<link rel="stylesheet" href="https://js.arcgis.com/3.25/esri/css/esri.css">
<style>
html, body, #map {
height: 100%;
margin: 0;
padding: 0;
}
</style>
<script src="https://js.arcgis.com/3.25/"></script>
<script>
var map;
require(["dojo/on","esri/map","esri/layers/FeatureLayer","esri/tasks/query", "esri/tasks/QueryTask","dojo/domReady!"], function(on,Map,FeatureLayer,Query,QueryTask) {
map = new Map("map", {
basemap: "topo",
center: [-100.45, 37.75],
zoom: 4
});
var featureLayer = new FeatureLayer("https://services.arcgis.com/P3ePLMYs2RVChkJx/ArcGIS/rest/services/World_Time_Zones/FeatureServer/0",{opacity:0.5});
map.addLayer(featureLayer);
map.on("click", function(evt){
var queryTask = new QueryTask("https://services.arcgis.com/P3ePLMYs2RVChkJx/ArcGIS/rest/services/World_Time_Zones/FeatureServer/0");
var query = new Query();
query.geometry = evt.mapPoint;
query.outFields = ["*"];
queryTask.on("complete", processResults);
queryTask.execute(query);
function processResults(results) {
var features = results.featureSet.features;
if (features.length > 0) {
alert('The zone offset for the clicked location is ' + features[0].attributes.ZONE);
};
};
});
});
</script>
</head>
<body>
<div><h2>Click on the map to get the time zone offset</h2></div>
<div id="map"></div>
</body>
</html>