diff --git a/epsiEDTtoICS.sh b/epsiEDTtoICS.sh
index 934331e..84d4117 100755
--- a/epsiEDTtoICS.sh
+++ b/epsiEDTtoICS.sh
@@ -64,6 +64,10 @@ function check_parameters_and_set_defaults() {
     log 2 "The directory (option -c or -p) “$(dirname "$OUTPUT")” is not writeable, or the filename is invalid"
     exit 2
   fi
+  if [ -n "$PATCH" -a "$OUTPUT" != - ] && ! [ -f "$OUTPUT" -a -r "$OUTPUT" -a -w "$OUTPUT" ]; then
+    log 2 "Patching (option -p) needs an existing r/w file; this one is not: $OUTPUT"
+    exit 3
+  fi
   if [ -n "$JOURNAL" ] && ! which systemd-cat >/dev/null; then
     JOURNAL=
     log 4 "Command “systemd-cat” not found"
@@ -211,6 +215,30 @@ function output_ical_week {
   | html_week_to_ical_week "$1" "$2"
 }
 
+# $1: offset (±) from today, to locate the week
+function dump_known_ical_week {
+  local offsetDayOfWeek=$(date +%u -d "${1}days")                # ∈ [1=monday … 5=friday]
+  local mon=$(date +%Y%m%d -d "$((${1}+1-$offsetDayOfWeek))days")
+  local tue=$(date +%Y%m%d -d "$((${1}+2-$offsetDayOfWeek))days")
+  local wed=$(date +%Y%m%d -d "$((${1}+3-$offsetDayOfWeek))days")
+  local thu=$(date +%Y%m%d -d "$((${1}+4-$offsetDayOfWeek))days")
+  local fri=$(date +%Y%m%d -d "$((${1}+5-$offsetDayOfWeek))days")
+
+  tr -d '\r' <"$OUTPUT" \
+  | awk -v week="^DTSTART:($mon|$tue|$wed|$thu|$fri)" '
+    function out() {
+      if (inWeek==1) {
+        printf("%s", day)
+        inWeek=0
+      }
+    }
+    /BEGIN:VEVENT/ { day=$0 "\n"; next }
+                   { day=day $0 "\n"   }
+    $0 ~ week      { inWeek=1          }
+    /END:VEVENT/   { out()             }
+  '
+}
+
 # &1: first offset of the school-year, and max offset of the same school-year, space-separated
 function get_first_and_max_offset_of_year() {
   local currentYear=$(date +%Y)
@@ -229,21 +257,46 @@ function get_first_and_max_offset_of_year() {
   printf '%s %s\n' $offset $offsetMax
 }
 
+# $1: offset (±) from today, to locate the week
+# exit status: 0 if $1 is in the current week
+function is_current_week() {
+  local offsetWeek=$(date +%G%V -d "${1}days")
+  local currentWeek=$(date +%G%V)
+  [ $offsetWeek -eq $currentWeek ]
+}
+
 function main() {
   check_parameters_and_set_defaults
   {
     local dtstamp=$(date -u +%Y%m%dT%H%M%SZ)
     local offset= offsetMax=
+    local step=-1 # -1: before patching; 0: patching time-span; 1: after patching
 
     read -r offset offsetMax < <(get_first_and_max_offset_of_year)
-    if [ -n "$PATCH" ]; then
-      offset=0
-      [ $MAXOFFSET -lt $offsetMax ] && offsetMax=$MAXOFFSET
-    fi
 
     output_ical_header
     while [ $offset -lt $offsetMax ]; do
-      output_ical_week $offset "$dtstamp"
+      if [ -n "$PATCH" ]; then
+        case $step in
+        -1)
+          if is_current_week $offset; then
+            MAXOFFSET=$((MAXOFFSET+offset))
+            step=0
+            continue
+          fi
+          dump_known_ical_week $offset ;;
+        0)
+          if [ $offset -ge $MAXOFFSET ]; then
+            step=1
+            continue
+          fi
+          output_ical_week $offset "$dtstamp" ;;
+        1)
+          dump_known_ical_week $offset ;;
+        esac
+      else
+        output_ical_week $offset "$dtstamp"
+      fi
       offset=$(($offset+7))
     done
     output_ical_footer