date: convert specific date to timestamp (#2697)

pages/common/date.md

@@ -17,3 +17,7 @@
 - Display a specific date (represented as a Unix timestamp) using the default format:
 
 `date -d @1473305798`
+
+- Convert a specific date to the Unix timestamp format:
+
+`date -d "{{2018-09-01 00:00}}" +%s --utc`