diff --git a/src/core/timer.c b/src/core/timer.c
index 5716c4f81d..70bcb56b83 100644
--- a/src/core/timer.c
+++ b/src/core/timer.c
@@ -431,6 +431,21 @@ static void timer_enter_waiting(Timer *t, bool time_change) {
                                 rebase_after_boot_time = true;
                         }
 
+                        if (b > ts.realtime) {
+                                /* The base we picked is in the future relative to the current realtime. This
+                                 * happens when the wall clock is set backwards while the timer is already
+                                 * waiting: the base was recorded from the old, later time. Feeding a future
+                                 * base to calendar_spec_next_usec() would schedule the next elapse relative to
+                                 * that stale time instead of now, so systemctl list-timers keeps showing the
+                                 * pre-adjustment elapse and the timer never catches up (see #6036). Clamp to
+                                 * now (minus the random offset, matching the "start from now" branch above) so
+                                 * we recalculate from the current time. */
+                                log_unit_debug(UNIT(t),
+                                               "Calendar timer base time %s is in the future, recalculating from the current time.",
+                                               FORMAT_TIMESTAMP(b));
+                                b = ts.realtime - random_offset;
+                        }
+
                         r = calendar_spec_next_usec(v->calendar_spec, b, &v->next_elapse);
                         if (r < 0)
                                 continue;
diff --git a/test/units/TEST-53-TIMER.clock-change.sh b/test/units/TEST-53-TIMER.clock-change.sh
new file mode 100755
index 0000000000..9a79494c13
--- /dev/null
+++ b/test/units/TEST-53-TIMER.clock-change.sh
@@ -0,0 +1,116 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+# Provides coverage for:
+#   - https://github.com/systemd/systemd/issues/6036
+#   - https://github.com/systemd/systemd/issues/24984
+
+# shellcheck source=test/units/util.sh
+. "$(dirname "$0")"/util.sh
+
+WAITING_UNIT="timer-clock-change-waiting-$RANDOM"
+MISSED_UNIT="timer-clock-change-missed-$RANDOM"
+MISSED_MARKER="/tmp/$MISSED_UNIT.ran"
+MISSED_STAMP="/var/lib/systemd/timers/stamp-$MISSED_UNIT.timer"
+
+START_REALTIME="$(date "+%s")"
+START_MONOTONIC="$(cut -d . -f 1 /proc/uptime)"
+
+at_exit() {
+    set +e
+
+    systemctl stop \
+        "$WAITING_UNIT.timer" \
+        "$WAITING_UNIT.service" \
+        "$MISSED_UNIT.timer" \
+        "$MISSED_UNIT.service"
+    systemctl clean --what=state "$MISSED_UNIT.timer"
+    rm -f \
+        "/run/systemd/system/$MISSED_UNIT.timer" \
+        "/run/systemd/system/$MISSED_UNIT.service" \
+        "$MISSED_MARKER" \
+        "$MISSED_STAMP"
+    systemctl daemon-reload
+
+    END_MONOTONIC="$(cut -d . -f 1 /proc/uptime)"
+    date --set="@$((START_REALTIME + END_MONOTONIC - START_MONOTONIC))"
+}
+
+trap at_exit EXIT
+
+timer_delta_s() {
+    local next_elapse next_elapse_s now
+
+    next_elapse="$(systemctl show -P NextElapseUSecRealtime "$1")"
+    next_elapse_s="$(date --date="$next_elapse" "+%s")"
+    now="$(date "+%s")"
+
+    TIMER_NEXT_ELAPSE="$next_elapse"
+    TIMER_NOW="$now"
+    TIMER_DELTA_S="$((next_elapse_s - now))"
+}
+
+assert_timer_due_within() {
+    local ok=0 unit="${1:?}" max_delta_s="${2:?}"
+
+    for _ in {1..50}; do
+        timer_delta_s "$unit"
+
+        if ((TIMER_DELTA_S >= 0 && TIMER_DELTA_S <= max_delta_s)); then
+            ok=1
+            break
+        fi
+
+        sleep .2
+    done
+
+    if ((ok == 0)); then
+        echo "Timer elapse outside of the expected window."
+        echo "  unit=$unit"
+        echo "  next_elapse=$TIMER_NEXT_ELAPSE"
+        echo "  now=$TIMER_NOW"
+        echo "  time_delta=$TIMER_DELTA_S"
+        exit 1
+    fi
+}
+
+: "A waiting calendar timer is recalculated after the clock is set backwards"
+
+date --set="+3 days"
+systemd-run --unit "$WAITING_UNIT" --on-calendar "*:0/15:0" true
+systemctl status "$WAITING_UNIT.timer"
+
+date --set="-3 days"
+assert_timer_due_within "$WAITING_UNIT.timer" 1200
+
+systemctl stop "$WAITING_UNIT.timer" "$WAITING_UNIT.service"
+
+: "A persistent calendar timer still catches up after time advances past a missed elapse"
+
+date --set="19:00:00"
+
+cat >"/run/systemd/system/$MISSED_UNIT.timer" <<EOF
+[Timer]
+OnCalendar=*-*-* 20:30:00
+Persistent=true
+AccuracySec=1ms
+EOF
+
+cat >"/run/systemd/system/$MISSED_UNIT.service" <<EOF
+[Service]
+Type=oneshot
+ExecStart=touch $MISSED_MARKER
+EOF
+
+systemctl daemon-reload
+mkdir -p "$(dirname "$MISSED_STAMP")"
+touch -d "yesterday 20:30:00" "$MISSED_STAMP"
+
+systemctl start "$MISSED_UNIT.timer"
+systemctl status "$MISSED_UNIT.timer"
+test ! -e "$MISSED_MARKER"
+
+date --set="tomorrow 09:00:00"
+timeout 30 bash -xec "until [[ -e '$MISSED_MARKER' ]]; do sleep .5; done"