docs: timezone - set default timezone

.gitignore

@@ -1,4 +1,5 @@
 .DS_Store
+.idea
 
 node_modules
 

docs/plugin/timezone.md

@@ -3,7 +3,7 @@ id: timezone
 title: Timezone
 ---
 
-Timezone adds `dayjs.tz` `.tz` `.tz.guess` APIs to parse or display between time zones.
+Timezone adds `dayjs.tz` `.tz` `.tz.guess` `.tz.setDefault` APIs to parse or display between time zones.
 
 ```javascript
 var utc = require('dayjs/plugin/utc') // dependent on utc plugin
@@ -16,4 +16,6 @@ dayjs.tz("2014-06-01 12:00", "America/New_York")
 dayjs("2014-06-01 12:00").tz("America/New_York")
 
 dayjs.tz.guess()
+
+dayjs.tz.setDefault("America/New_York")
 ```

docs/timezone/set-default-timezone.md

@@ -0,0 +1,27 @@
+---
+id: set-default-timezone
+title: Set Default Timezone
+---
+
+Change default timezone from local time zone to your custom timezone.
+
+You can still custom a different timezone in a specific `dayjs` object.
+
+@>Timezone
+```javascript
+dayjs.extend(utc)
+dayjs.extend(timezone)
+
+dayjs.tz.setDefault("America/New_York")
+
+// The same behavior with dayjs.tz("2014-06-01 12:00", "America/New_York")
+dayjs.tz("2014-06-01 12:00")  // 2014-06-01T12:00:00-04:00
+
+// use another timezone
+dayjs.tz("2014-06-01 12:00", "Asia/Tokyo")  // 2014-06-01T12:00:00+09:00
+
+// reset timezone
+dayjs.tz.setDefault()
+```
+
+Notice: `dayjs.tz.setDefault` will not affect existing `dayjs` objects.

website/sidebars.json

@@ -166,7 +166,8 @@
       "timezone/timezone",
       "timezone/parsing-in-zone",
       "timezone/converting-to-zone",
-      "timezone/guessing-user-timezone"
+      "timezone/guessing-user-timezone",
+      "timezone/set-default-timezone"
     ]
   },
   "docs-other": {