Add example for command n,g,p and grid, update date now examples (#4622)

docs/commands/date_now.md

@@ -16,3 +16,13 @@ Get the current date and display it in a given format string.
 ```shell
 > date now | date format "%Y-%m-%d %H:%M:%S"
 ```
+
+Get the time duration from 2019-04-30 to now
+```shell
+> (date now) - 2019-05-01
+```
+
+Get the time duration since a more accurate time
+```shell
+> (date now) - 2019-05-01T04:12:05.20+08:00
+```