diff --git a/plugins/ssl/ssl-certificate-expiry b/plugins/ssl/ssl-certificate-expiry
index 9295380a..7c69face 100755
--- a/plugins/ssl/ssl-certificate-expiry
+++ b/plugins/ssl/ssl-certificate-expiry
@@ -41,6 +41,7 @@ Copyright (C) 2017 Olivier Mehani <shtrom+munin@ssji.net>
 
 =cut
 
+# shellcheck disable=SC1090
 . "${MUNIN_LIBDIR}/plugins/plugin.sh"
 
 if [ "${MUNIN_DEBUG}" = 1 ]; then
@@ -54,6 +55,44 @@ if [ "${HOSTPORT}" != "${0}" ] \
 	services="${HOSTPORT}"
 fi
 
+
+# Read data including a certificate from stdin and output the (fractional) number of days left
+# until the expiry of this certificate. The output is empty if parsing failed.
+parse_valid_days_from_certificate() {
+    local input_data
+    local valid_until_string
+    local valid_until_epoch
+    local now_epoch
+    local input_data
+    input_data=$(cat)
+    if echo "$input_data" | grep -q -- "-----BEGIN CERTIFICATE-----"; then
+        valid_until_string=$(echo "$input_data" | openssl x509 -noout -enddate \
+            | grep "^notAfter=" | cut -f 2 -d "=")
+        if [ -n "$valid_until_string" ]; then
+            valid_until_epoch=$(date --date="$valid_until_string" +%s)
+            if [ -n "$valid_until_epoch" ]; then
+                now_epoch=$(date +%s)
+                # calculate the number of days left
+                echo "$valid_until_epoch" "$now_epoch" | awk '{ print(($1 - $2) / (24 * 3600)); }'
+            fi
+        fi
+    fi
+}
+
+
+print_expire_days() {
+    local host="$1"
+    local port="$2"
+
+    # Wrap IPv6 addresses in square brackets
+    echo "$host" | grep -q ':' && host="[$host]"
+
+    echo "" | openssl s_client -CApath /etc/ssl/certs \
+            -servername "$host" -connect "${host}:${port}" 2>/dev/null \
+        | parse_valid_days_from_certificate
+}
+
+
 case $1 in
     config)
 
@@ -72,40 +111,17 @@ case $1 in
 	;;
 esac
 
-get_expire()
-{
-    SITE="$(echo "${1}" | sed 's/_.*//')"
-    PORT="$(echo "${1}" | sed 's/.*_//')"
-
-    VAR="$(clean_fieldname "$1")"
-    if [ "$PORT" = "$SITE" ]; then
-	PORT=443
-    fi
-    if echo "$SITE" | grep -q ':'; then
-	# Wrap IPv6 addresses in square brackets
-        SITE="[${SITE}]"
-    fi
-
-    CERT=$(echo "" | openssl s_client -CApath /etc/ssl/certs -servername "${SITE}" -connect "${SITE}:${PORT}" 2>/dev/null);
-
-    if echo "${CERT}" | grep -q -- "-----BEGIN CERTIFICATE-----"; then
-	echo "${CERT}" \
-		| openssl x509 -noout -enddate \
-		| awk -F= 'BEGIN {
-			split("Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec", month, " ");
-			for (i=1; i<=12; i++)
-				mdigit[month[i]] = i;
-		}
-		/notAfter/ {
-			split($0,a,"="); split(a[2],b," "); split(b[3],time,":");
-			datetime=b[4] " " mdigit[b[1]] " " b[2] " " time[1] " " time[2] " " time[3];
-			days=(mktime(datetime)-systime())/86400;
-			print "VAR.value " days;
-		}' \
-		| sed "s/VAR/${VAR}/g"
-    fi
-}
 
 for service in $services; do
-    get_expire "$service"
+    if echo "$service" | grep -q "_"; then
+        host=$(echo "$service" | cut -f 1 -d "_")
+        port=$(echo "$service" | cut -f 2 -d "_")
+    else
+        host=$service
+        port=443
+    fi
+    fieldname="$(clean_fieldname "$service")"
+    valid_days=$(print_expire_days "$host" "$port")
+    [ -z "$valid_days" ] && valid_days="U"
+    printf "%s.value %s\n" "$fieldname" "$valid_days"
 done
diff --git a/plugins/ssl/ssl_ b/plugins/ssl/ssl_
index 76e7686f..496453f1 100755
--- a/plugins/ssl/ssl_
+++ b/plugins/ssl/ssl_
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # -*- sh -*-
 
 : << =cut
@@ -26,15 +26,43 @@ Copyright (C) 2013 Patrick Domack <patrickdk@patrickdk.com>
 
 =cut
 
+# shellcheck disable=SC1090
 . "$MUNIN_LIBDIR/plugins/plugin.sh"
 
 ARGS=${0##*ssl_}
-SITE=${ARGS/_*/}
-PORT=${ARGS##*_}
-if [ "$PORT" = "$SITE" ]; then
-	PORT=443
+if echo "$ARGS" | grep -q "_"; then
+    SITE=$(echo "$ARGS" | cut -f 1 -d "_")
+    PORT=$(echo "$ARGS" | cut -f 2 -d "_")
+else
+    SITE=$ARGS
+    PORT=443
 fi
 
+
+# Read data including a certificate from stdin and output the (fractional) number of days left
+# until the expiry of this certificate. The output is empty if parsing failed.
+parse_valid_days_from_certificate() {
+    local input_data
+    local valid_until_string
+    local valid_until_epoch
+    local now_epoch
+    local input_data
+    input_data=$(cat)
+    if echo "$input_data" | grep -q -- "-----BEGIN CERTIFICATE-----"; then
+        valid_until_string=$(echo "$input_data" | openssl x509 -noout -enddate \
+            | grep "^notAfter=" | cut -f 2 -d "=")
+        if [ -n "$valid_until_string" ]; then
+            valid_until_epoch=$(date --date="$valid_until_string" +%s)
+            if [ -n "$valid_until_epoch" ]; then
+                now_epoch=$(date +%s)
+                # calculate the number of days left
+                echo "$valid_until_epoch" "$now_epoch" | awk '{ print(($1 - $2) / (24 * 3600)); }'
+            fi
+        fi
+    fi
+}
+
+
 case $1 in
     config)
 
@@ -53,18 +81,7 @@ esac
 
 cert=$(echo "" | openssl s_client -CApath /etc/ssl/certs -servername "${SITE}" -connect "${SITE}:${PORT}" 2>/dev/null);
 
-if [[ "${cert}" = *"-----BEGIN CERTIFICATE-----"* ]]; then
-	echo "${cert}" \
-		| openssl x509 -noout -enddate \
-		| awk -F= 'BEGIN {
-			split("Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec", month, " ");
-			for (i=1; i<=12; i++)
-				mdigit[month[i]] = i;
-		}
-		/notAfter/ {
-			split($0,a,"="); split(a[2],b," "); split(b[3],time,":");
-			datetime=b[4] " " mdigit[b[1]] " " b[2] " " time[1] " " time[2] " " time[3];
-			days=(mktime(datetime)-systime())/86400;
-			print "expire.value " days;
-		}'
-fi
+days_left=$(echo "$cert" | parse_valid_days_from_certificate)
+[ -n "$days_left" ] || days_left="U"
+
+printf "expire.value %s\n" "$days_left"