diff --git a/Koha/Library/Calendar/Exception.pm b/Koha/Library/Calendar/Exception.pm
index 598c23d020..fb118db204 100644
--- a/Koha/Library/Calendar/Exception.pm
+++ b/Koha/Library/Calendar/Exception.pm
@@ -21,6 +21,7 @@ use Modern::Perl;
 
 use Koha::Caches;
 use Koha::Database;
+use Koha::Exceptions;
 
 use base qw(Koha::Object);
 
@@ -38,12 +39,20 @@ Overloaded store method that clears the holidays cache on insert.
 
 Only flushes on insert (new exception), not on title/description updates,
 because the holidays cache only stores dates and their open/closed status.
+An update that changes C<date> in place would desync the cache with no
+warning, so that is rejected outright - callers must delete and re-add
+instead.
 
 =cut
 
 sub store {
     my ($self) = @_;
 
+    Koha::Exceptions::BadParameter->throw(
+        error     => 'Cannot change date on an existing closure exception; delete and re-add instead',
+        parameter => 'date'
+    ) if $self->in_storage && $self->_result->is_column_changed('date');
+
     my $flush = !$self->in_storage;
 
     $self = $self->SUPER::store;
diff --git a/Koha/Library/Calendar/SingleClosure.pm b/Koha/Library/Calendar/SingleClosure.pm
index 079d712d28..fbe807effd 100644
--- a/Koha/Library/Calendar/SingleClosure.pm
+++ b/Koha/Library/Calendar/SingleClosure.pm
@@ -21,6 +21,7 @@ use Modern::Perl;
 
 use Koha::Caches;
 use Koha::Database;
+use Koha::Exceptions;
 
 use base qw(Koha::Object);
 
@@ -38,12 +39,20 @@ Overloaded store method that clears the holidays cache on insert.
 
 Only flushes on insert (new closure), not on title/description updates,
 because the holidays cache only stores dates and their open/closed status.
+An update that changes C<date> in place would desync the cache with no
+warning, so that is rejected outright - callers must delete and re-add
+instead.
 
 =cut
 
 sub store {
     my ($self) = @_;
 
+    Koha::Exceptions::BadParameter->throw(
+        error     => 'Cannot change date on an existing single closure; delete and re-add instead',
+        parameter => 'date'
+    ) if $self->in_storage && $self->_result->is_column_changed('date');
+
     my $flush = !$self->in_storage;
 
     $self = $self->SUPER::store;
diff --git a/t/db_dependent/Koha/Library/Calendar.t b/t/db_dependent/Koha/Library/Calendar.t
index 2708892d9e..e7b7cfccff 100755
--- a/t/db_dependent/Koha/Library/Calendar.t
+++ b/t/db_dependent/Koha/Library/Calendar.t
@@ -19,7 +19,7 @@
 
 use Modern::Perl;
 
-use Test::More tests => 7;
+use Test::More tests => 8;
 use Test::Exception;
 use Test::NoWarnings;
 
@@ -339,3 +339,35 @@ subtest 'add_weekly_closure validates weekday' => sub {
 
     $schema->storage->txn_rollback;
 };
+
+subtest 'SingleClosure/Exception store() rejects an in-place date change' => sub {
+
+    plan tests => 4;
+
+    $schema->storage->txn_begin;
+
+    my $library  = $builder->build_object( { class => 'Koha::Libraries' } );
+    my $calendar = Koha::Library::Calendar->new( branchcode => $library->branchcode );
+
+    my $single = $calendar->add_single_closure( { date => '2027-08-01', title => 'Staff day', description => '' } );
+    throws_ok {
+        $single->date('2027-08-02')->store;
+    }
+    'Koha::Exceptions::BadParameter', 'SingleClosure->store dies when date is changed in place';
+
+    $single->discard_changes;
+    $single->title('Staff day (renamed)')->store;
+    is( $single->title, 'Staff day (renamed)', 'SingleClosure->store still allows a title-only update' );
+
+    my $exception = $calendar->add_exception( { date => '2027-08-03', title => 'Special open', description => '' } );
+    throws_ok {
+        $exception->date('2027-08-04')->store;
+    }
+    'Koha::Exceptions::BadParameter', 'Exception->store dies when date is changed in place';
+
+    $exception->discard_changes;
+    $exception->title('Special open (renamed)')->store;
+    is( $exception->title, 'Special open (renamed)', 'Exception->store still allows a title-only update' );
+
+    $schema->storage->txn_rollback;
+};