--- loncom/html/adm/helper/newslot.helper	2019/03/02 16:25:52	1.30.6.1
+++ loncom/html/adm/helper/newslot.helper	2020/04/08 20:54:58	1.30.6.2
@@ -208,11 +208,34 @@
         delete($helper->{'VARS'}{'startreserve'});
         delete($helper->{'VARS'}{'endreserve'});
         delete($helper->{'VARS'}{'maxspace'});
-        delete($helper->{'VARS'}{'startunique'});
-        delete($helper->{'VARS'}{'endunique'});
       </exec>
+      <message>
+         </p><p>Period of time when this slot can only be uniquely assigned:<br />&nbsp;&nbsp;&nbsp;Start:
+      </message>
+      <date variable="startunique" hoursminutes="1" anytime="1">
+        <defaultvalue>
+          my $default=&{$helper->{DATA}{origslot}}('uniqueperiod','anytime');
+          if ($default eq 'anytime') { return 'anytime' };
+          if (ref($default)) { return $default->[0]; }
+          return 'anytime';
+        </defaultvalue>
+      </date>
+      <message><br />&nbsp;&nbsp;&nbsp;End: </message>
+      <date variable="endunique" hoursminutes="1" anytime="1">
+        <defaultvalue>
+          my $default=&{$helper->{DATA}{origslot}}('uniqueperiod','anytime');
+          if ($default eq 'anytime') { return 'anytime' };
+          if (ref($default)) { return $default->[1]; }
+          return 'anytime';
+        </defaultvalue>
+        <validator>
+           if (defined($val) && $val < $helper->{'VARS'}{'startunique'}) {
+               return 'End time must be later than the start time.';
+           }
+           return undef;
+        </validator>
+      </date>
     </condition>
-
     <message>
       </p><p>Proctored access:
     </message>