Fix issue #2037 - modifying a dayjs instance created with plugin timezone in UTC corrupts it

package.json

@@ -7,7 +7,7 @@
   "scripts": {
     "test": "TZ=Pacific/Auckland npm run test-tz && TZ=Europe/London npm run test-tz && TZ=America/Whitehorse npm run test-tz && npm run test-tz && jest",
     "test-tz": "date && jest test/timezone.test --coverage=false",
-    "lint": "./node_modules/.bin/eslint src/* test/* build/*",
+    "lint": "npx eslint src/* test/* build/*",
     "prettier": "prettier --write \"docs/**/*.md\"",
     "babel": "cross-env BABEL_ENV=build babel src --out-dir esm --copy-files && node build/esm",
     "build": "cross-env BABEL_ENV=build node build && npm run size",

src/plugin/timezone/index.js

@@ -12,7 +12,7 @@ const typeToPos = {
 // Cache time-zone lookups from Intl.DateTimeFormat,
 // as it is a *very* slow method.
 const dtfCache = {}
-const getDateTimeFormat = (timezone, options = {}) => {
+const getDateTimeFormat = (timezone, options) => {
   const timeZoneName = options.timeZoneName || 'short'
   const key = `${timezone}|${timeZoneName}`
   let dtf = dtfCache[key]
@@ -97,8 +97,13 @@ export default (o, c, d) => {
     const date = this.toDate()
     const target = date.toLocaleString('en-US', { timeZone: timezone })
     const diff = Math.round((date - new Date(target)) / 1000 / 60)
-    let ins = d(target, { locale: this.$L }).$set(MS, this.$ms)
-      .utcOffset((-Math.round(date.getTimezoneOffset() / 15) * 15) - diff, true)
+    const offset = -date.getTimezoneOffset() - diff
+    if (offset === 0) {
+      return this.utc(keepLocalTime)
+    }
+    let ins = d(target)
+      .$set(MS, this.$ms)
+      .utcOffset(offset, true)
     if (keepLocalTime) {
       const newOffset = ins.utcOffset()
       ins = ins.add(oldOffset - newOffset, MIN)
@@ -128,10 +133,13 @@ export default (o, c, d) => {
   d.tz = function (input, arg1, arg2) {
     const parseFormat = arg2 && arg1
     const timezone = arg2 || arg1 || defaultTimezone
-    const previousOffset = tzOffset(+d(), timezone)
-    if (typeof input !== 'string') {
-      // timestamp number || js Date || Day.js
-      return d(input).tz(timezone)
+    const previousOffset = tzOffset(+d(input, parseFormat), timezone)
+    // To differentiate date only string (e.g. yy-mm-dd) from date string with negative
+    // 2-digit offset (hour offset zz, e.g. yy-mm-zz) we require at least 8 characters
+    // before offset (i.e. yy-mm-dd-zz)
+    if ((typeof input !== 'string') || /.{8,}[+-]\d\d:?(\d\d)?$|Z$/i.test(input)) {
+      // timestamp number || js Date || Day.js || input string with offset (e.g. -03:00)
+      return d(input, parseFormat).tz(timezone)
     }
     const localTs = d.utc(input, parseFormat).valueOf()
     const [targetTs, targetOffset] = fixOffset(localTs, previousOffset, timezone)

src/plugin/utc/index.js

@@ -20,7 +20,6 @@ function offsetFromString(value = '') {
   return indicator === '+' ? totalOffsetInMinutes : -totalOffsetInMinutes
 }
 
-
 export default (option, Dayjs, dayjs) => {
   const proto = Dayjs.prototype
   dayjs.utc = function (date) {