Fix YAML date coercion (#438)

lib/utils/serverless-utils/cloudformation-schema.js

@@ -43,7 +43,13 @@ const createSchema = () => {
   const types = functionNames.flatMap((functionName) =>
     ['mapping', 'scalar', 'sequence'].map((kind) => yamlType(functionName, kind))
   );
-  return yaml.DEFAULT_SCHEMA.extend(types);
+  const implicitTypes = yaml.DEFAULT_SCHEMA.implicit.filter(
+    (type) => type.tag !== 'tag:yaml.org,2002:timestamp'
+  );
+  return new yaml.Schema({
+    implicit: implicitTypes,
+    explicit: yaml.DEFAULT_SCHEMA.explicit,
+  }).extend(types);
 };
 
 module.exports = createSchema();

test/unit/lib/configuration/read.test.js

@@ -46,6 +46,31 @@ describe('test/unit/lib/configuration/read.test.js', () => {
     });
   });
 
+  it('should preserve date-shaped YAML values as strings', async () => {
+    configurationPath = 'serverless.yml';
+    await fsp.writeFile(
+      configurationPath,
+      [
+        'service: test-date-strings',
+        'provider:',
+        '  name: aws',
+        '  unquotedDate: 2020-12-12',
+        "  quotedDate: '2020-12-12'",
+        '  explicitlyTaggedDate: !!str 2020-12-12',
+        '',
+      ].join('\n')
+    );
+    expect(await readConfiguration(configurationPath)).to.deep.equal({
+      service: 'test-date-strings',
+      provider: {
+        name: 'aws',
+        unquotedDate: '2020-12-12',
+        quotedDate: '2020-12-12',
+        explicitlyTaggedDate: '2020-12-12',
+      },
+    });
+  });
+
   it('should read "serverless.json"', async () => {
     configurationPath = 'serverless.json';
     const configuration = {