Use Intl.DateTimeFormat to fix DST offsets

package.json

@@ -31,5 +31,8 @@
     "eslint": "^5.16.0",
     "mocha": "^6.1.4",
     "nyc": "^14.0.0"
+  },
+  "engines": {
+    "node": ">=8.0.0"
   }
 }

src/tz-offset.js

@@ -1,15 +1,54 @@
 'use strict';
-const offsets = require('../generated/offsets.json');
 
 module.exports = (() => {
 
-    const offsetOf = (timezone) => {
-        const offset = offsets[timezone];
-        if (offset != undefined && offset != null) {
-            return offset;
-        } else {
-            throw Error('Invalid timezone ' + timezone);
+    const _DTFCache = {};
+
+    function _getDTF(timezone) {
+        if (!_DTFCache[timezone]) {
+            try {
+                const dtf = new Intl.DateTimeFormat('en-US', {
+                    hour12: false,
+                    timeZone: timezone,
+                    year: 'numeric',
+                    month: '2-digit',
+                    day: '2-digit',
+                    hour: '2-digit',
+                    minute: '2-digit',
+                    second: '2-digit'
+                });
+                _DTFCache[timezone] = dtf;
+            } catch(err) {
+                throw new Error('Invalid timezone ' + timezone);
+            }
         }
+        return _DTFCache[timezone];
+    }
+
+    function _fromStringFormat(dtf, date) {
+        const formatted = dtf.format(date).replace(/\u200E/g, ''),
+            parsed = formatted.match(/(\d+)\/(\d+)\/(\d+),? (\d+):(\d+):(\d+)/),
+            [, month, day, year, hour, minute, second] = parsed.map(n => parseInt(n, 10));
+        return new Date(Date.UTC(year, month - 1, day, hour, minute, second));
+    }
+
+    function _fromPartsFormat(dtf, date) {
+        const formatted = dtf.formatToParts(date);
+        return new Date(Date.UTC(
+            parseInt(formatted[4].value, 10),
+            parseInt(formatted[0].value, 10) - 1,
+            parseInt(formatted[2].value, 10),
+            parseInt(formatted[6].value, 10),
+            parseInt(formatted[8].value, 10),
+            parseInt(formatted[10].value, 10)
+        ));
+    }
+
+    const offsetOf = (timezone, date) => {
+        if (!date) date = new Date();
+
+        const timestamp = date.getTime();
+        return ((timestamp - (timestamp % 1000)) - timeAt(date, timezone)) / 60000;
     };
 
     const removeOffset = (date) => {
@@ -18,9 +57,10 @@ module.exports = (() => {
     };
 
     const timeAt = (date, timezone) => {
-        const timeUtc = removeOffset(date);
-        const offset = offsetOf(timezone) * -60000;
-        return new Date(timeUtc + offset);
+        const dtf = _getDTF(timezone);
+        return dtf.formatToParts
+            ? _fromPartsFormat(dtf, date)
+            : _fromStringFormat(dtf, date);
     };
 
     return {

test/tz-offset-test.js

@@ -5,16 +5,24 @@ const { assert } = require('chai');
 process.env.TZ = 'America/Sao_Paulo';
 
 describe('tz-offset', () => {
-    it('should return 180 to America/Sao_Paulo', () => {
-        assert.equal(tzOffset.offsetOf('America/Sao_Paulo'), 180);
+    it('should return 180 to America/Sao_Paulo (non DST)', () => {
+        assert.equal(tzOffset.offsetOf('America/Sao_Paulo', new Date(2019, 7, 1)), 180);
     });
 
-    it('should return -60 to Africa/Algiers', () => {
+    it('should return 120 to America/Sao_Paulo (DST)', () => {
+        assert.equal(tzOffset.offsetOf('America/Sao_Paulo', new Date(2019, 0, 1)), 120);
+    });
+
+    it('should return -60 to Africa/Algiers (Doesn\'t have DST)', () => {
         assert.equal(tzOffset.offsetOf('Africa/Algiers'), -60);
     });
 
-    it('should return 0 to Europe/London', () => {
-        assert.equal(tzOffset.offsetOf('Europe/London'), 0);
+    it('should return 0 to Europe/London (non DST)', () => {
+        assert.equal(tzOffset.offsetOf('Europe/London', new Date(2019, 0, 1)), 0);
+    });
+
+    it('should return -60 to Europe/London (DST)', () => {
+        assert.equal(tzOffset.offsetOf('Europe/London', new Date(2019, 7, 1)), -60);
     });
 
     it('should fail with invalid timezone', () => {
@@ -35,4 +43,12 @@ describe('tz-offset', () => {
         const result = tzOffset.timeAt(date, 'Etc/UTC');
         assert.equal(result.toString(), expectedDate.toString());
     });
+
+    it('should return the time at zone (using fromStringFormat)', () => {
+        Intl.DateTimeFormat.prototype.formatToParts = undefined;
+        const date = new Date(2018, 8, 20, 0, 0, 0, 0);
+        const expectedDate = new Date(2018, 8, 20, 3, 0, 0, 0);
+        const result = tzOffset.timeAt(date, 'Etc/UTC');
+        assert.equal(result.toString(), expectedDate.toString());
+    });
 });