Allow printing the date in compact ISO 8601 format. E.g.:

date    :	Wed Nov 11 14:55:04 MST 2009
date -i :	20091111T145504


The 8601 format allows rc scripts to create compact timestamps for log
entries, is easier to manipulate with sed/grep/etc., and is a natural
key for sort(1). ('date -n' is compact and sortable, but isn't human
readable or regexp friendly.)

Reference: /n/sources/patch/maybe/date-iso8601
Date: Wed Nov 11 23:06:17 CET 2009
Signed-off-by: lyndon@orthanc.ca

--- /sys/src/cmd/date.c	Wed Nov 11 22:54:32 2009
+++ /sys/src/cmd/date.c	Thu Nov 12 23:26:27 2009
@@ -1,17 +1,20 @@
 #include <u.h>
 #include <libc.h>
 
-int uflg, nflg;
+int iflg, nflg, uflg;
 
 void
 main(int argc, char *argv[])
 {
 	ulong now;
+	int tzhr, tzmin;
+	struct Tm *tm;
 
 	ARGBEGIN{
+	case 'i':	iflg = 1; break;
 	case 'n':	nflg = 1; break;
 	case 'u':	uflg = 1; break;
-	default:	fprint(2, "usage: date [-un] [seconds]\n"); exits("usage");
+	default:	fprint(2, "usage: date [-inu] [seconds]\n"); exits("usage");
 	}ARGEND
 
 	if(argc == 1)
@@ -21,6 +24,17 @@
 
 	if(nflg)
 		print("%ld\n", now);
+	else if(iflg){
+		if(uflg)
+			tm = gmtime(now);
+		else
+			tm = localtime(now);
+		tzhr = tm->tzoff / 3600;
+		tzmin = (abs(tm->tzoff) - (abs(tzhr) * 3600)) / 60;
+		print("%0.4d%0.2d%0.2dT%0.2d%0.2d%0.2d%+.2d%.2d\n",
+			tm->year + 1900, tm->mon + 1, tm->mday,
+			tm->hour, tm->min, tm->sec, tzhr, tzmin);
+	}
 	else if(uflg)
 		print("%s", asctime(gmtime(now)));
 	else
--- /sys/man/1/date	Wed Nov 11 22:54:36 2009
+++ /sys/man/1/date	Wed Nov 11 22:54:34 2009
@@ -11,19 +11,22 @@
 .br
 .B clock
 .SH DESCRIPTION
-Print the date, in the format
+Print the date in the format
 .PP
 .B
 	Tue Aug 16 17:03:52 CDT 1977
 .PP
 The options are
 .TP
-.B -u
-Report Greenwich Mean Time (GMT) rather than local time.
+.B -i
+Print the date in compact ISO 8601 format.
 .TP
 .B -n
 Report the date as the number of seconds since the
 epoch, 00:00:00 GMT, January 1, 1970.
+.TP
+.B -u
+Report Greenwich Mean Time (GMT) rather than local time.
 .PP
 The conversion from Greenwich Mean Time to local time depends on the
 .B $timezone
@@ -33,7 +36,7 @@
 If the optional argument
 .I seconds
 is present, it is used as the time to convert rather than
-the real time.
+the present time.
 .PP
 .I Clock
 draws a simple analog clock in its window.