diff --git a/config/dot/quickshell/services/CalendarAgenda.qml b/config/dot/quickshell/services/CalendarAgenda.qml
index a9e334d..0d49db2 100644
--- a/config/dot/quickshell/services/CalendarAgenda.qml
+++ b/config/dot/quickshell/services/CalendarAgenda.qml
@@ -169,7 +169,9 @@ Singleton {
 
     function openCalendar(date: date): void {
         const target = date && !Number.isNaN(date.getTime()) ? date : root.selectedDate;
-        Quickshell.execDetached(["gnome-calendar", "--date", root.dateKey(target)]);
+        // GNOME Calendar passes this value to g_date_time_new_from_iso8601(),
+        // which requires a time and timezone rather than a bare YYYY-MM-DD.
+        Quickshell.execDetached(["gnome-calendar", "--date", target.toISOString()]);
     }
 
     function join(event: var): void {
diff --git a/tests/quickshell/calendar-agenda-contract.sh b/tests/quickshell/calendar-agenda-contract.sh
index 2677681..21e7e2a 100755
--- a/tests/quickshell/calendar-agenda-contract.sh
+++ b/tests/quickshell/calendar-agenda-contract.sh
@@ -48,6 +48,8 @@ for component in AgendaPane AgendaEvent NotificationSummary; do
     rg -q "${component} 1.0 ${component}.qml" config/dot/quickshell/modules/datemenu/qmldir \
         || fail "$component is not registered"
 done
+rg -q 'gnome-calendar.*--date.*toISOString' config/dot/quickshell/services/CalendarAgenda.qml \
+    || fail 'GNOME Calendar date handoff is not a complete ISO-8601 timestamp'
 
 qs ipc call calendar-agenda fixture upcoming >/dev/null
 jq -e '.capsuleVisible == true and .capsuleText == "12m"' \