Improve IANAZone.offset performance

src/zones/IANAZone.js

@@ -1,20 +1,36 @@
 import { formatOffset, parseZoneInfo, isUndefined, objToLocalTS } from "../impl/util.js";
 import Zone from "../zone.js";
 
+let hasLongOffset = true;
+try {
+  new Intl.DateTimeFormat("en-US", { timeZoneName: "longOffset" });
+} catch (e) {
+  hasLongOffset = false;
+}
+
 let dtfCache = {};
 function makeDTF(zone) {
   if (!dtfCache[zone]) {
-    dtfCache[zone] = new Intl.DateTimeFormat("en-US", {
-      hour12: false,
-      timeZone: zone,
-      year: "numeric",
-      month: "2-digit",
-      day: "2-digit",
-      hour: "2-digit",
-      minute: "2-digit",
-      second: "2-digit",
-      era: "short",
-    });
+    dtfCache[zone] = new Intl.DateTimeFormat(
+      "en-US",
+      hasLongOffset
+        ? {
+            timeZone: zone,
+            timeZoneName: "longOffset",
+            year: "numeric",
+          }
+        : {
+            hour12: false,
+            timeZone: zone,
+            year: "numeric",
+            month: "2-digit",
+            day: "2-digit",
+            hour: "2-digit",
+            minute: "2-digit",
+            second: "2-digit",
+            era: "short",
+          }
+    );
   }
   return dtfCache[zone];
 }
@@ -52,6 +68,28 @@ function partsOffset(dtf, date) {
   return filled;
 }
 
+function longOffset(dtf, ts) {
+  let formatted;
+
+  try {
+    formatted = dtf.format(ts);
+  } catch (e) {
+    return NaN;
+  }
+
+  const idx = formatted.search(/GMT([+-][0-9][0-9]:[0-9][0-9](:[0-9][0-9])?)?/);
+  const sign = formatted.charCodeAt(idx + 3);
+
+  if (isNaN(sign)) return 0;
+
+  return (
+    (44 - sign) *
+    (Number(formatted.slice(idx + 4, idx + 6)) * 60 +
+      Number(formatted.slice(idx + 7, idx + 9)) +
+      Number(formatted.slice(idx + 10, idx + 12) / 60))
+  );
+}
+
 let ianaZoneCache = {};
 /**
  * A zone identified by an IANA identifier, like America/New_York
@@ -145,6 +183,8 @@ export default class IANAZone extends Zone {
 
   /** @override **/
   offset(ts) {
+    if (hasLongOffset) return longOffset(makeDTF(this.name), ts);
+
     const date = new Date(ts);
 
     if (isNaN(date)) return NaN;