perf: reuse calendar timezone formatters (Closes #1285)

frontend/week-calendar-import.js

@@ -1,5 +1,6 @@
 function createWeekCalendarImport() {
   const maxBytes=1024*1024;
+  const zonedInstantFormatters=new Map(),zonePartFormatters=new Map(),supportedTimeZones=new Map();
   function clock(value) {
     const match=/^(\d{2}):(\d{2})$/.exec(String(value||''));
     if(!match)return null;
@@ -19,9 +20,11 @@ function createWeekCalendarImport() {
     const match=/^(\d{4})(\d{2})(\d{2})T(\d{2})(\d{2})(\d{2})$/.exec(value||'');
     if(!match)return null;
     try {
-      const wanted=match.slice(1).map(Number),formatter=new Intl.DateTimeFormat('en-CA',{
+      const wanted=match.slice(1).map(Number);
+      if(!zonedInstantFormatters.has(timeZone))zonedInstantFormatters.set(timeZone,new Intl.DateTimeFormat('en-CA',{
         timeZone,year:'numeric',month:'2-digit',day:'2-digit',hour:'2-digit',minute:'2-digit',second:'2-digit',hourCycle:'h23',
-      });
+      }));
+      const formatter=zonedInstantFormatters.get(timeZone);
       let result=Date.UTC(wanted[0],wanted[1]-1,wanted[2],wanted[3],wanted[4],wanted[5]);
       for(let attempt=0;attempt<2;attempt+=1) {
         const shown=Object.fromEntries(formatter.formatToParts(new Date(result)).map(part=>[part.type,part.value]));
@@ -33,15 +36,18 @@ function createWeekCalendarImport() {
   }
   function supportedTimeZone(timeZone) {
     if(!timeZone)return true;
-    try {new Intl.DateTimeFormat('en-US',{timeZone}).format(0);return true;}
-    catch(error) {return false;}
+    if(supportedTimeZones.has(timeZone))return supportedTimeZones.get(timeZone);
+    try {new Intl.DateTimeFormat('en-US',{timeZone}).format(0);supportedTimeZones.set(timeZone,true);return true;}
+    catch(error) {supportedTimeZones.set(timeZone,false);return false;}
   }
   function zoneParts(value,timeZone) {
     const date=new Date(value);
     if(!timeZone)return {year:date.getFullYear(),month:date.getMonth()+1,day:date.getDate(),weekday:date.getDay(),
       hour:date.getHours(),minute:date.getMinutes(),second:date.getSeconds()};
-    const formatter=new Intl.DateTimeFormat('en-US',{timeZone,year:'numeric',month:'2-digit',day:'2-digit',
-      hour:'2-digit',minute:'2-digit',second:'2-digit',weekday:'short',hourCycle:'h23'});
+    if(!zonePartFormatters.has(timeZone))zonePartFormatters.set(timeZone,new Intl.DateTimeFormat('en-US',{
+      timeZone,year:'numeric',month:'2-digit',day:'2-digit',hour:'2-digit',minute:'2-digit',second:'2-digit',weekday:'short',hourCycle:'h23',
+    }));
+    const formatter=zonePartFormatters.get(timeZone);
     const parts=Object.fromEntries(formatter.formatToParts(date).map(part=>[part.type,part.value]));
     return {year:Number(parts.year),month:Number(parts.month),day:Number(parts.day),
       weekday:['Sun','Mon','Tue','Wed','Thu','Fri','Sat'].indexOf(parts.weekday),hour:Number(parts.hour),