diff --git a/README.md b/README.md
index 3229451..5dd1fd7 100644
--- a/README.md
+++ b/README.md
@@ -54,6 +54,9 @@ client.find_instance_record(instance_record_id: 'some-instance-record-id')
 client.find_instance_record(instance_record_hrid: 'some-instance-record-hrid')
 client.find_source_record(instance_record_id: 'some-instance-record-id')
 client.find_source_record(instance_record_hrid: 'some-instance-record-hrid')
+client.find_source_marc_records(modified_since: '2025-01-01T00:00:00Z') { |marc_record_hash| }
+client.find_source_marc_records(with_965_value: '965abc') { |marc_record_hash| }
+client.find_source_marc_records(modified_since: '2025-01-01T00:00:00Z', with_965_value: '965abc') { |marc_record_hash| }
 
 # Convert a FOLIO MARC source record to a marc gem MARC::Record object:
 source_record = client.find_source_record(instance_record_id: 'some-instance-record-id')
diff --git a/lib/folio_api_client/finders.rb b/lib/folio_api_client/finders.rb
index 463e6ab..c1fc4f8 100644
--- a/lib/folio_api_client/finders.rb
+++ b/lib/folio_api_client/finders.rb
@@ -90,8 +90,8 @@ def find_source_record(instance_record_id: nil, instance_record_hrid: nil)
       source_record_search_results['sourceRecords'].first
     end
 
-    def find_source_marc_records(modified_since, has_965hyacinth: false, &block)
-      query = marc_records_query(modified_since: modified_since, has_965hyacinth: has_965hyacinth)
+    def find_source_marc_records(modified_since: nil, with_965_value: nil, &block)
+      query = marc_records_query(modified_since: modified_since, with_965_value: with_965_value)
 
       loop do
         response = self.get('search/instances', query)
@@ -105,6 +105,8 @@ def find_source_marc_records(modified_since, has_965hyacinth: false, &block)
     def process_marc_for_instance(instances, &block)
       instances.each do |instance|
         source_record = find_source_record(instance_record_id: instance['id'])
+        next if source_record.nil? # Occasionally, we find an instance record without a source record.  Skip these.
+
         marc_content = source_record.dig('parsedRecord', 'content')
         yield(marc_content) if marc_content && block
       end
@@ -133,17 +135,22 @@ def location_record_query(code: nil)
             'Missing query field.  Must supply a code.'
     end
 
-    def marc_records_query(modified_since: nil, has_965hyacinth: false)
+    def marc_records_query(modified_since: nil, with_965_value: nil) # rubocop:disable Metrics/MethodLength
       params = { limit: 100, offset: 0 }
 
-      if modified_since.nil? && !has_965hyacinth
+      if modified_since.nil? && with_965_value.nil?
         raise FolioApiClient::Exceptions::MissingQueryFieldError,
-              'Missing query field.  Must supply either modified_since or has_965hyacinth=true.'
+              'Missing query field.  Must supply either modified_since or with_965_value.'
+      end
+
+      if modified_since && !modified_since.match(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z$/)
+        raise ArgumentError,
+              %(Invalid format for modified_since argument. Must be a date string like "2025-10-03T16:49:00Z".)
       end
 
       query_parts = []
       query_parts << "metadata.updatedDate>=\"#{modified_since}\"" if modified_since
-      query_parts << 'identifiers.value="965hyacinth"' if has_965hyacinth
+      query_parts << %(identifiers.value="#{with_965_value}") if with_965_value
 
       params[:query] = query_parts.join(' and ')
       params
diff --git a/spec/folio_api_client/finders_spec.rb b/spec/folio_api_client/finders_spec.rb
index 452db47..1103c07 100644
--- a/spec/folio_api_client/finders_spec.rb
+++ b/spec/folio_api_client/finders_spec.rb
@@ -333,6 +333,7 @@
 
   describe '#find_source_marc_records' do
     let(:modified_since) { '2025-01-01T00:00:00Z' }
+    let(:value_965) { '965hyacinth' }
     let(:marc_content) { { 'fields' => [{ '001' => '12345' }] } }
     let(:instance_record) { { 'id' => 'instance-123' } }
     let(:instances_response) { { 'totalRecords' => 1, 'instances' => [instance_record] } }
@@ -350,50 +351,56 @@
 
       it 'yields MARC content for each source record' do
         yielded_records = []
-        instance.find_source_marc_records(modified_since) { |record| yielded_records << record }
+        instance.find_source_marc_records(modified_since: modified_since) { |record| yielded_records << record }
         expect(yielded_records).to eq([marc_content])
       end
+
+      it 'raises an exception if the given modified_since parameter is an invalid format' do
+        expect {
+          instance.find_source_marc_records(modified_since: 'banana') { |_| }
+        }.to raise_error(ArgumentError)
+      end
     end
 
-    context 'with has_965hyacinth parameter' do
+    context 'with with_965_value parameter' do
       before do
         allow(instance).to receive(:get).with(
-          'search/instances', { query: 'identifiers.value="965hyacinth"', limit: 100, offset: 0 }
+          'search/instances', { query: %(identifiers.value="#{value_965}"), limit: 100, offset: 0 }
         ).and_return(instances_response)
       end
 
       it 'uses the correct query' do
-        instance.find_source_marc_records(nil, has_965hyacinth: true) { |_| }
+        instance.find_source_marc_records(with_965_value: value_965) { |_| }
         expect(instance).to have_received(:get).with(
           'search/instances', { query: 'identifiers.value="965hyacinth"', limit: 100, offset: 0 }
         )
       end
     end
 
-    context 'with both parameters' do
+    context 'with modified_since and with_965_value parameters' do
       before do
         allow(instance).to receive(:get).with(
           'search/instances', {
-            query: "metadata.updatedDate>=\"#{modified_since}\" and identifiers.value=\"965hyacinth\"",
+            query: %(metadata.updatedDate>="#{modified_since}" and identifiers.value="#{value_965}"),
             limit: 100, offset: 0
           }
         ).and_return(instances_response)
       end
 
       it 'combines both filters' do
-        instance.find_source_marc_records(modified_since, has_965hyacinth: true) { |_| }
+        instance.find_source_marc_records(modified_since: modified_since, with_965_value: value_965) { |_| }
         expect(instance).to have_received(:get).with(
           'search/instances', {
-            query: "metadata.updatedDate>=\"#{modified_since}\" and identifiers.value=\"965hyacinth\"",
+            query: %(metadata.updatedDate>="#{modified_since}" and identifiers.value="#{value_965}"),
             limit: 100, offset: 0
           }
         )
       end
     end
 
-    it 'raises error when no parameters provided' do
+    it 'raises an error when no parameters provided' do
       expect {
-        instance.find_source_marc_records(nil) { |_| }
+        instance.find_source_marc_records { |_| }
       }.to raise_error(FolioApiClient::Exceptions::MissingQueryFieldError)
     end
   end