Add date RFC 3339 (#3902)

pages/common/date.md

@@ -21,3 +21,7 @@
 - Convert a specific date to the Unix timestamp format:
 
 `date -d "{{2018-09-01 00:00}}" +%s --utc`
+
+- Display the current date using the RFC-3339 format (`YYYY-MM-DD hh:mm:ss TZ`):
+
+`date --rfc-3339=s`