Nagios' check_http tool will alert you to SSL certificate
expiry.  Unfortunately it uses the ambiguous US method of
specifying the date (MM/DD/YY): so e.g. 

    OK - Certificate will expire on 11/12/2006 10:24.

Means 'the 12th of November' rather than 'the 11th of
December', which is what the same string means in most other
locales.

This patch changes this to a more sane YYYY-MM-DD format.

                         Jon Dowland <jon.dowland@ncl.ac.uk>
                             Wed, 01 Nov 2006 15:56:24 +0000

--- /usr/local/src/nagios-plugins-1.4.2/plugins/check_http.c	2006-06-19 14:07:11.000000000 +0100
+++ /usr/local/nagios/current/check_http.c	2006-11-01 15:57:44.000000000 +0000
@@ -1351,9 +1351,9 @@
 
 	days_left = (mktime (&stamp) - time (NULL)) / 86400;
 	snprintf
-		(timestamp, 17, "%02d/%02d/%04d %02d:%02d",
-		 stamp.tm_mon + 1,
-		 stamp.tm_mday, stamp.tm_year + 1900, stamp.tm_hour, stamp.tm_min);
+		(timestamp, 17, "%04d-%02d-%02d %02d:%02d",
+		 stamp.tm_year + 1900, stamp.tm_mon + 1, stamp.tm_mday,
+		 stamp.tm_hour, stamp.tm_min);
 
 	if (days_left > 0 && days_left <= days_till_exp) {
 		printf (_("WARNING - Certificate expires in %d day(s) (%s).\n"), days_left, timestamp);