Formatting Dates Only in ArcGIS Online web map popups.

app.js

require(["esri/arcgis/utils"], function (arcgisUtils) {
  //...
  
  // functionality to fix dates
  var oneDay = 24 * 60 * 60 * 1000;
  function timeFix(value) {
    var dateShift, val;
    if (value === undefined || value === null) {
      return value;
    }
    val = value.getTime === "function" ? value.getTime() : value;
    // assumption: we want to shift datetimes where the time is midnight UTC (down to the millisecond);
    if (val % oneDay === 0) {
      // if the date has a time of midnight UTC, we shift it by the timezone offset
      dateShift = new Date(val).getTimezoneOffset() * 60 * 1000;
      return val + dateShift;
    }
  }
  
  // create the map.
  arcgisUtils.createMap("/* insert web map id here */", "mapdiv", {}).then(function (response) {
    var map = response.map;
    // do stuff 
    
    if (map.infoWindow) {
      map.infoWindow.on("set-features", function () {
        // for each selected feature in the infoWindow popup...
        map.infoWindow.features.forEach(function (feature) {
          // get the layer from the feature
          var layer = feature._graphicsLayer || feature._layer || null;
          // look at the fields in the layer (if present).
          if (layer && layer.fields && layer.fields.length) {
            layer.fields.forEach(function(field) {
              // if the field type is a date field...
              if (field.type === "esriFieldTypeDate") {
                // fix the time.
                feature.attributes[field.name] = timeFix(feature.attributes[field.name]);
              }
            });
          }
        });
      })
    }
  });
});