Add unix epoch commands

These differ between Linux and OS X so the pages needed to be split.

pages/linux/date.md

@@ -9,3 +9,11 @@
 - Display the date in UTC and ISO 8601 format:
 
 `date -u +"%Y-%m-%dT%H:%M:%SZ"`
+
+- Display the number of seconds since the unix epoch:
+
+`date +%s`
+
+- Convert a unix epoch timestamp to a date:
+
+`date -d @1473305798`

pages/osx/date.md

@@ -0,0 +1,19 @@
+# date
+
+> Set or display the system date.
+
+- Display the date using the default locale:
+
+`date +"%c"`
+
+- Display the date in UTC and ISO 8601 format:
+
+`date -u +"%Y-%m-%dT%H:%M:%SZ"`
+
+- Display the number of seconds since the unix epoch:
+
+`date +%s`
+
+- Juggle a timestamp representing seconds since the Unix expoch to a date:
+
+`date -j -r 1473305798`