diff --git a/lbplanner/classes/helpers/slot_helper.php b/lbplanner/classes/helpers/slot_helper.php
index 70a52e98..bde076f7 100644
--- a/lbplanner/classes/helpers/slot_helper.php
+++ b/lbplanner/classes/helpers/slot_helper.php
@@ -283,9 +283,11 @@ public static function get_reservations_for_user(int $userid): array {
     public static function filter_reservations_for_recency(array $reservations): array {
         $sentryspan = sentry_helper::span_start(__FUNCTION__, ['count_in' => count($reservations)]);
 
+        $now = new DateTimeImmutable('now', new DateTimeZone('UTC'));
+
         $goodeggs = [];
         foreach ($reservations as $reservation) {
-            if (!$reservation->is_outdated()) {
+            if (!$reservation->is_endpast($now)) {
                 array_push($goodeggs, $reservation);
             }
         }
diff --git a/lbplanner/classes/model/reservation.php b/lbplanner/classes/model/reservation.php
index cdd43b73..fe04f27c 100644
--- a/lbplanner/classes/model/reservation.php
+++ b/lbplanner/classes/model/reservation.php
@@ -164,14 +164,23 @@ public function get_datetime_end(): DateTimeImmutable {
     }
 
     /**
-     * Calculates whether the reservation is outdated
+     * Calculates whether the start of this reservation has passed
+     * @param \DateTimeInterface $now
      *
-     * @return DateTimeImmutable
+     * @return bool
      */
-    public function is_outdated(): bool {
-        $utctz = new DateTimeZone('UTC');
-        $yesterday = new DateTimeImmutable('yesterday', $utctz);
-        return $yesterday->diff($this->date)->invert;
+    public function is_startpast($now): bool {
+        return $this->is_endpast($now) || ($now->diff($this->get_datetime())->invert === 1);
+    }
+
+    /**
+     * Calculates whether the end of this reservation has passed
+     * @param \DateTimeInterface $now
+     *
+     * @return bool
+     */
+    public function is_endpast($now): bool {
+        return $now->diff($this->get_datetime_end())->invert === 1;
     }
 
     /**
diff --git a/lbplanner/lang/de/local_lbplanner.php b/lbplanner/lang/de/local_lbplanner.php
index 60e4b96e..dfe6a44d 100644
--- a/lbplanner/lang/de/local_lbplanner.php
+++ b/lbplanner/lang/de/local_lbplanner.php
@@ -61,6 +61,7 @@
 $string['err_plan_changeaccess_ofowner'] = 'Kann Berechtigungen ders BesitzerIns nicht ändern';
 $string['err_plan_changeaccess_self'] = 'Kann eigene Berechtigungen nicht ändern';
 $string['err_plan_changeaccess_toowner'] = 'Kann Berechtigungen nicht auf BesitzerIn ändern';
+$string['err_reserv_current'] = 'Schon laufende Slots können nicht reserviert werden';
 $string['err_reserv_past'] = 'Vergangene Termine können nicht reserviert werden';
 $string['err_reserv_slotfull'] = 'Slot ist schon voll';
 $string['err_reserv_studentalrin'] = 'SchülerIn hat bereits eine Reservierung für diesen Slot';
diff --git a/lbplanner/lang/en/local_lbplanner.php b/lbplanner/lang/en/local_lbplanner.php
index d7173fcf..10b9baca 100644
--- a/lbplanner/lang/en/local_lbplanner.php
+++ b/lbplanner/lang/en/local_lbplanner.php
@@ -61,6 +61,7 @@
 $string['err_plan_changeaccess_ofowner'] = 'Cannot change permissions for the plan owner';
 $string['err_plan_changeaccess_self'] = 'Cannot change own permissions';
 $string['err_plan_changeaccess_toowner'] = 'Cannot change permission to owner';
+$string['err_reserv_current'] = 'Can\'t reserve slot that has already started';
 $string['err_reserv_past'] = 'Can\'t reserve date in the past';
 $string['err_reserv_slotfull'] = 'Slot is already full';
 $string['err_reserv_studentalrin'] = 'Student already has a reservation for this slot';
diff --git a/lbplanner/services/slots/book_reservation.php b/lbplanner/services/slots/book_reservation.php
index 2e49839c..d4fe48f0 100644
--- a/lbplanner/services/slots/book_reservation.php
+++ b/lbplanner/services/slots/book_reservation.php
@@ -85,23 +85,19 @@ public static function book_reservation(int $slotid, string $date, int $userid):
         );
 
         $utctz = new DateTimeZone('UTC');
-        $now = new DateTimeImmutable('today', $utctz);
+        $now = new DateTimeImmutable('now', $utctz);
+
         $dateobj = DateTimeImmutable::createFromFormat("Y-m-d", $date, $utctz);
         if ($dateobj === false) {
             throw new \moodle_exception(get_string('err_dateformat', 'local_lbplanner', $date));
         }
-        $td = $now->diff($dateobj);
-
-        if ($td->invert === 1) {
-            throw new \moodle_exception(get_string('err_reserv_past', 'local_lbplanner'));
-        }
 
         $maxdays = null;
         $student = null;
-
         $curuserid = intval($USER->id);
+        $issupervisor = $userid === $curuserid;
 
-        if ($userid === $curuserid) {
+        if ($issupervisor) {
             // Student reserving slot for themself.
 
             $maxdays = config_helper::get_slot_futuresight();
@@ -114,12 +110,20 @@ public static function book_reservation(int $slotid, string $date, int $userid):
             $student = core_user::get_user($userid, '*', MUST_EXIST);
         }
 
-        if ($td->days > $maxdays) {
+        $slot = slot_helper::get_slot($slotid);
+        $reservation = new reservation(0, $slotid, $dateobj, $userid, $curuserid);
+        $reservation->set_slot($slot);
+
+        // Check if reservation is not too old, nor too far into the future.
+        $td = $now->diff($dateobj);
+        if ($reservation->is_endpast($now)) {
+            throw new \moodle_exception(get_string('err_reserv_past', 'local_lbplanner'));
+        } else if (!$issupervisor && $reservation->is_startpast($now)) {
+            throw new \moodle_exception(get_string('err_reserv_current', 'local_lbplanner'));
+        } else if ($td->days > $maxdays) {
             throw new \moodle_exception(get_string('err_reserv_toofuture', 'local_lbplanner', $maxdays));
         }
 
-        $slot = slot_helper::get_slot($slotid);
-
         // Check if user has access to slot.
         if (count(slot_helper::filter_slots_for_user([$slot], $student)) === 0) {
             throw new \moodle_exception(get_string('err_reserv_studentnoaccess', 'local_lbplanner'));
@@ -137,9 +141,6 @@ public static function book_reservation(int $slotid, string $date, int $userid):
             throw new \moodle_exception(get_string('err_reserv_slotfull', 'local_lbplanner'));
         }
 
-        $reservation = new reservation(0, $slotid, $dateobj, $userid, $curuserid);
-        $reservation->set_slot($slot);
-
         // Check if user is already in a different slot at the same time.
         $overlapreservations = [];
         $existingreservations = slot_helper::get_reservations_for_user($userid);
diff --git a/lbplanner/services/slots/unbook_reservation.php b/lbplanner/services/slots/unbook_reservation.php
index d984797b..e50dd4c0 100644
--- a/lbplanner/services/slots/unbook_reservation.php
+++ b/lbplanner/services/slots/unbook_reservation.php
@@ -17,6 +17,7 @@
 namespace local_lbplanner_services;
 
 use DateTimeImmutable;
+use DateTimeZone;
 use core_external\{external_api, external_function_parameters, external_value};
 use local_lbplanner\helpers\{slot_helper, notifications_helper};
 use local_lbplanner\enums\NOTIF_TRIGGER;
@@ -71,10 +72,10 @@ public static function unbook_reservation(int $reservationid, bool $nice): void
         $userid = intval($USER->id);
 
         $reservation = slot_helper::get_reservation($reservationid);
-        $now = new DateTimeImmutable();
+        $now = new DateTimeImmutable('now', new DateTimeZone('UTC'));
 
-        $endpast = $now->diff($reservation->get_datetime_end())->invert === 1;
-        $startpast = $endpast || ($now->diff($reservation->get_datetime())->invert === 1);
+        $endpast = $reservation->is_endpast($now);
+        $startpast = $reservation->is_startpast($now);
 
         if ($userid === $reservation->userid) {
             if ($endpast) {