Add support for Unix timestamps.

cmd/utc/README

@@ -1,5 +1,9 @@
 utc: convert times to UTC
 
+Usage:	utc [-f format] [-o format] [-q] [-t] [-u] [-z zone] [time(s)...]
+	utc -h | utc help
+
+
 utc converts times to UTC. If no arguments are provided, prints the
 current time in UTC. If the only argument provided is "-", utc reads
 newline-separated timestamps from standard input. If the argument is
@@ -8,8 +12,6 @@ the input and output timezones are the same (e.g., the local time zone
 is UTC), a warning message will be printed on standard error. This can
 be suppressed with the -q option.
 
-Usage: utc [-f format] [-h] [-o format] [-q] [-u] [-z zone] [time(s)...]
-
 Flags:
 
 	-f format	Go timestamp format for input times. See the Go docs
@@ -20,22 +22,29 @@ Flags:
 
 	-h		Print this help message.
 
-	-o format	Go timestamp format for outputting times. It uses
-			the same format as the '-f' argument.
+	-o format       Go timestamp format for outputting times.
+			It uses the same format as the '-f' argument.
 
 			Default value: 2006-01-02 15:04 MST
 
 	-q		Suppress the timezone check warning message.
 
-	-u		Timestamps are in UTC format and should be converted
-			to the timezone specified by the -z argument (which
-			defaults to 'Local'). Note that this isn't particularly
-			useful with no arguments.
+	-t              Input times are Unix timestamps. Use with
+			-u to convert the timestamp to the timezone
+			specified by the -z option (which defaults
+			to Local).
+
+	-u              Timestamps are in UTC format and should be
+			converted to the timezone specified by the
+			-z argument (which defaults to 'Local'). Note
+			that this isn't particularly useful with
+			no arguments.
 
 	-z zone		Text form of the time zone; this can be in short
-			time zone abbreviation (e.g. MST) or a location
-			(e.g. America/Los_Angeles). This has no effect when
-			printing the current time.
+			time zone abbreviation (e.g. MST) or a
+			location (e.g. America/Los_Angeles). This
+			has no effect when printing the current
+			time.
 
 			Default value: Local
 
@@ -69,6 +78,15 @@ PST8PDT time zone):
 	+ Using a different output format:
 	  $ utc -o '2006-01-02T15:03:04-0700' '2016-06-14 21:30' 
 	  2016-06-14T21:09:30-0700 = 2016-06-15T04:04:30+0000
+	+ Converting a Unix timestamp to a UTC time:
+	  $ utc -t 1466052938
+	  2016-06-15 21:55 PDT = 2016-06-16 04:55 UTC
+	+ Converting a Unix timestamp to local time:
+	  $ utc -t -u 1466052938
+	  2016-06-16 04:55 UTC = 2016-06-15 21:55 PDT
+	+ Converting a Unix timestamp to EST:
+	  $ utc -t -u -z EST 1466052938
+	  2016-06-16 04:55 UTC = 2016-06-15 23:55 EST
 	+ Example of the warning message when running utc on a machine
 	  where the local time zone is UTC:
 	  $ utc