diff --git a/app/assets/javascripts/controllers/admin/calendar.js.erb b/app/assets/javascripts/controllers/admin/calendar.js.erb
index dba9d8ef1..be492e2a0 100644
--- a/app/assets/javascripts/controllers/admin/calendar.js.erb
+++ b/app/assets/javascripts/controllers/admin/calendar.js.erb
@@ -283,12 +283,12 @@ Application.Controllers.controller('AdminCalendarController', ['$scope', '$state
         return uiCalendarConfig.calendars.calendar.fullCalendar('unselect');
       }
 
-      // check that the selected slot is an N-hours multiple (ie. not decimal)
+      // check that the selected slot is an multiple of SLOT_MULTIPLE (ie. not decimal)
       const slots = Math.trunc((end.valueOf() - start.valueOf()) / (60 * 1000)) / SLOT_MULTIPLE;
       if (!Number.isInteger(slots)) {
-        // otherwise, round it to nearest decimal
-        const nearest = Math.round(slots) * SLOT_MULTIPLE;
-        end = moment(start).add(nearest, 'minutes');
+        // otherwise, round it to upper decimal
+        const upper = Math.ceil(slots) * SLOT_MULTIPLE;
+        end = moment(start).add(upper, 'minutes');
       }
 
       // then we open a modal window to let the admin specify the slot type
@@ -507,21 +507,25 @@ Application.Controllers.controller('CreateEventModalController', ['$scope', '$ui
 
       Tag.query().$promise.then(function (data) { $scope.tags = data; });
 
-      // When we configure a machine availability, do not let the user change the end time, as the total
-      // time must be dividable by 60 minutes (base slot duration). For training availabilities, the user
+      // When we configure a machine/space availability, do not let the user change the end time, as the total
+      // time must be dividable by Fablab.slotDuration minutes (base slot duration). For training availabilities, the user
       // can configure any duration as it does not matters.
       $scope.$watch('availability.available_type', function (newValue, oldValue, scope) {
         if ((newValue === 'machines') || (newValue === 'space')) {
           $scope.endDateReadOnly = true;
-          const diff = moment($scope.end).diff($scope.start, 'hours'); // the result is rounded down by moment.js
-          $scope.end = moment($scope.start).add(diff, 'hours').toDate();
+          const slots = Math.trunc(($scope.end.valueOf() - $scope.start.valueOf()) / (60 * 1000)) / Fablab.slotDuration;
+          if (!Number.isInteger(slots)) {
+            // otherwise, round it to upper decimal
+            const upper = Math.ceil(slots) * Fablab.slotDuration;
+            $scope.end = moment($scope.start).add(upper, 'minutes').toDate();
+          }
           return $scope.availability.end_at = $scope.end;
         } else {
           return $scope.endDateReadOnly = false;
         }
       });
 
-      // When the start date is changed, if we are configuring a machine availability,
+      // When the start date is changed, if we are configuring a machine/space availability,
       // maintain the relative length of the slot (ie. change the end time accordingly)
       $scope.$watch('start', function (newValue, oldValue, scope) {
       // for machine or space availabilities, adjust the end time
@@ -542,7 +546,7 @@ Application.Controllers.controller('CreateEventModalController', ['$scope', '$ui
       // Maintain consistency between the end time and the date object in the availability object
       return $scope.$watch('end', function (newValue, oldValue, scope) {
       // we prevent the admin from setting the end of the availability before its begining
-        if (moment($scope.start).add(1, 'hour').isAfter(newValue)) {
+        if (moment($scope.start).add(Fablab.slotDuration, 'minutes').isAfter(newValue)) {
           $scope.end = oldValue;
         }
         // update availability object
diff --git a/doc/environment.md b/doc/environment.md
index 594c9cf17..4fcfc5ada 100644
--- a/doc/environment.md
+++ b/doc/environment.md
@@ -108,6 +108,8 @@ This is useful if you have your own invoicing system and you want to prevent Fab
 
 Machine and space availabilities are divided in multiple slots of the duration set by this variable.
 Default value is 60 minutes (1 hour).
+
+⚠ You should not change this value during the application lifecycle!
 <a name="DEFAULT_MAIL_FROM"></a>
 
     DEFAULT_MAIL_FROM