fix($shared-utils): fix date parse logic for permalinks (#2181)

This addresses the way the date is parsed, which, when it's a string is causing the input for the Date constructor to assume the date to be in ISO 8601 format (UTC), and when it's a Date, is always UTC.

packages/@vuepress/shared-utils/src/getPermalink.ts

@@ -12,6 +12,38 @@ interface PermalinkOption {
 function removeLeadingSlash (path: string) {
   return path.replace(/^\//, '')
 }
+
+function toUtcTime (date: string | Date): number {
+  let year = 1970
+  let month = 0
+  let day = 1
+
+  if (typeof date === 'string') {
+    const [
+      yearStr,
+      monthStr,
+      dayStr
+    ] = date.split('-')
+
+    year = parseInt(yearStr, 10)
+    month = parseInt(monthStr, 10) - 1
+    day = parseInt(dayStr, 10)
+  } else if (date instanceof Date) {
+    // If `date` is an instance of Date,
+    // it's because it was parsed from the frontmatter
+    // by js-yaml, which always assumes UTC
+    return date.getTime()
+  }
+
+  return Date.UTC(year, month, day)
+}
+
+function addTzOffset (utc: number): Date {
+  const utcDate = new Date(utc)
+
+  return new Date(utc + utcDate.getTimezoneOffset() * 60 * 1000)
+}
+
 // e.g.
 // filename: docs/_posts/evan you.md
 // content: # yyx 990803
@@ -33,7 +65,7 @@ export = function getPermalink ({
   }
   slug = encodeURI(slug)
 
-  const d = new Date(date)
+  const d = addTzOffset(toUtcTime(date))
   const year = d.getFullYear()
   const iMonth = d.getMonth() + 1
   const iDay = d.getDate()