diff --git a/lib/classes/yaml-parser.js b/lib/classes/yaml-parser.js
index b23b0c8e09..58c1a69263 100644
--- a/lib/classes/yaml-parser.js
+++ b/lib/classes/yaml-parser.js
@@ -3,6 +3,7 @@
 const path = require('path');
 const { pathToFileURL } = require('url');
 const yaml = require('js-yaml');
+const cloudformationSchema = require('../utils/serverless-utils/cloudformation-schema');
 const isPlainObject = require('type/plain-object/is');
 const ServerlessError = require('../serverless-error');
 const { isExternalRefAccessDeniedError } = require('./yaml-parser/external-ref-errors');
@@ -64,7 +65,9 @@ const loadExternalDocument = (documentUrl, state) => {
     state.documents.set(
       documentUrl,
       readExternalDocument(documentUrl, state.externalRefs).then((document) =>
-        yaml.load(Buffer.isBuffer(document) ? document.toString('utf8') : String(document))
+        yaml.load(Buffer.isBuffer(document) ? document.toString('utf8') : String(document), {
+          schema: cloudformationSchema,
+        })
       )
     );
   }
diff --git a/lib/utils/fs/parse.js b/lib/utils/fs/parse.js
index 8dc98cd571..f6d635abfd 100644
--- a/lib/utils/fs/parse.js
+++ b/lib/utils/fs/parse.js
@@ -4,34 +4,12 @@ const jc = require('json-cycle');
 const yaml = require('js-yaml');
 const cloudformationSchema = require('../serverless-utils/cloudformation-schema');
 
-const loadYaml = (contents, options) => {
-  let data;
-  let error;
-  try {
-    data = yaml.load(contents.toString(), options || {});
-  } catch (exception) {
-    error = exception;
-  }
-  return { data, error };
-};
-
 function parse(filePath, contents) {
   // Auto-parse JSON
   if (filePath.endsWith('.json') || filePath.endsWith('.tfstate')) {
     return jc.parse(contents);
   } else if (filePath.endsWith('.yml') || filePath.endsWith('.yaml')) {
-    const options = {
-      filename: filePath,
-    };
-    let result = loadYaml(contents.toString(), options);
-    if (result.error && result.error.name === 'YAMLException') {
-      options.schema = cloudformationSchema;
-      result = loadYaml(contents.toString(), options);
-    }
-    if (result.error) {
-      throw result.error;
-    }
-    return result.data;
+    return yaml.load(contents.toString(), { filename: filePath, schema: cloudformationSchema });
   }
   return contents.toString().trim();
 }
diff --git a/lib/utils/serverless-utils/README.md b/lib/utils/serverless-utils/README.md
index b57d781e52..8b2ca39337 100644
--- a/lib/utils/serverless-utils/README.md
+++ b/lib/utils/serverless-utils/README.md
@@ -26,6 +26,10 @@ Source of truth:
 
 Notes:
 
+- `cloudformation-schema.js` intentionally diverges from upstream: it removes
+  implicit YAML timestamp resolution and re-registers `!!timestamp` as an
+  explicit type (https://github.com/oss-serverless/osls/issues/438). Preserve
+  this when re-syncing.
 - `config.js` is a locally owned fork. It intentionally keeps the synchronous
   `get('frameworkId')` and `get('meta.created_at')` lookups that Bref v2/v3 use
   for best-effort telemetry if a future compatibility shim routes
diff --git a/lib/utils/serverless-utils/cloudformation-schema.js b/lib/utils/serverless-utils/cloudformation-schema.js
index ac02b47dd8..b5afb71421 100644
--- a/lib/utils/serverless-utils/cloudformation-schema.js
+++ b/lib/utils/serverless-utils/cloudformation-schema.js
@@ -43,7 +43,15 @@ const createSchema = () => {
   const types = functionNames.flatMap((functionName) =>
     ['mapping', 'scalar', 'sequence'].map((kind) => yamlType(functionName, kind))
   );
-  return yaml.DEFAULT_SCHEMA.extend(types);
+  // Drop implicit timestamps so date-shaped plain scalars (e.g. an IAM policy
+  // `Version: 2012-10-17`) stay strings; an explicit `!!timestamp` tag still constructs a Date
+  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, yaml.types.timestamp],
+  }).extend(types);
 };
 
 module.exports = createSchema();
diff --git a/test/unit/lib/classes/yaml-parser.test.js b/test/unit/lib/classes/yaml-parser.test.js
index 99e1ad3903..488fd47792 100644
--- a/test/unit/lib/classes/yaml-parser.test.js
+++ b/test/unit/lib/classes/yaml-parser.test.js
@@ -108,6 +108,23 @@ describe('YamlParser', () => {
         .to.equal('bar');
     });
 
+    it('should parse date-shaped values and shorthand tags in referenced files', () => {
+      const tmpDirPath = getTmpDirPath();
+
+      serverless.utils.writeFileSync(
+        path.join(tmpDirPath, 'ref.yml'),
+        'date: 2012-10-17\nref: !Ref Topic\n'
+      );
+
+      serverless.utils.writeFileSync(path.join(tmpDirPath, 'test.yml'), {
+        main: { $ref: './ref.yml' },
+      });
+
+      return expect(serverless.yamlParser.parse(path.join(tmpDirPath, 'test.yml')))
+        .to.eventually.have.property('main')
+        .to.deep.equal({ date: '2012-10-17', ref: { Ref: 'Topic' } });
+    });
+
     it('should leave same-document refs in the root file untouched', async () => {
       const tmpFilePath = getTmpFilePath('same-document.yml');
 
diff --git a/test/unit/lib/configuration/read.test.js b/test/unit/lib/configuration/read.test.js
index 8c72450a74..755996073c 100644
--- a/test/unit/lib/configuration/read.test.js
+++ b/test/unit/lib/configuration/read.test.js
@@ -46,6 +46,35 @@ 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',
+        '  unquotedDateTime: 2020-12-12T00:00:00Z',
+        '  spacedDateTime: 2020-12-12 00:00:00',
+        '',
+      ].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',
+        unquotedDateTime: '2020-12-12T00:00:00Z',
+        spacedDateTime: '2020-12-12 00:00:00',
+      },
+    });
+  });
+
   it('should read "serverless.json"', async () => {
     configurationPath = 'serverless.json';
     const configuration = {
diff --git a/test/unit/lib/configuration/variables/sources/file.test.js b/test/unit/lib/configuration/variables/sources/file.test.js
index bca3920010..2e15768101 100644
--- a/test/unit/lib/configuration/variables/sources/file.test.js
+++ b/test/unit/lib/configuration/variables/sources/file.test.js
@@ -16,6 +16,7 @@ describe('test/unit/lib/configuration/variables/sources/file.test.js', () => {
       yaml: '${file(file.yaml)}',
       yml: '${file(file.yml)}',
       json: '${file(file.json)}',
+      dateString: '${file(file-date.yml):date}',
       tfstate: '${file(file.tfstate)}',
       js: '${file(file.js)}',
       cjs: '${file(file.cjs)}',
@@ -87,6 +88,9 @@ describe('test/unit/lib/configuration/variables/sources/file.test.js', () => {
   it('should resolve "json" file sources', () =>
     expect(configuration.json).to.deep.equal({ result: 'json' }));
 
+  it('should resolve date-shaped values as strings', () =>
+    expect(configuration.dateString).to.equal('2012-10-17'));
+
   it('should resolve "tfstate" file sources', () =>
     expect(configuration.tfstate).to.deep.equal({ result: 'tfstate' }));
 
diff --git a/test/unit/lib/configuration/variables/sources/fixture/file-date.yml b/test/unit/lib/configuration/variables/sources/fixture/file-date.yml
new file mode 100644
index 0000000000..8dc03c590d
--- /dev/null
+++ b/test/unit/lib/configuration/variables/sources/fixture/file-date.yml
@@ -0,0 +1 @@
+date: 2012-10-17
diff --git a/test/unit/lib/utils/fs/parse.test.js b/test/unit/lib/utils/fs/parse.test.js
index 7de6aec510..61b15e45c2 100644
--- a/test/unit/lib/utils/fs/parse.test.js
+++ b/test/unit/lib/utils/fs/parse.test.js
@@ -114,6 +114,12 @@ describe('#parse()', () => {
     });
   });
 
+  it('should keep date-shaped values as strings and support explicit timestamp tags', () => {
+    const obj = parse('anything.yml', 'date: 2012-10-17\ntagged: !!timestamp 2020-12-12');
+    expect(obj.date).to.equal('2012-10-17');
+    expect(obj.tagged).to.be.instanceOf(Date);
+  });
+
   it('should parse YAML without shorthand syntax', () => {
     const tmpFilePath = 'anything.yml';
     const fileContents = 'Item:\n  Fn::Join:\n  - ""\n  - - "arn:aws:s3::"\n    - !Ref MyBucket';
diff --git a/test/unit/lib/utils/serverless-utils/cloudformation-schema.test.js b/test/unit/lib/utils/serverless-utils/cloudformation-schema.test.js
new file mode 100644
index 0000000000..1b8ed2d8a8
--- /dev/null
+++ b/test/unit/lib/utils/serverless-utils/cloudformation-schema.test.js
@@ -0,0 +1,20 @@
+'use strict';
+
+const expect = require('chai').expect;
+const yaml = require('js-yaml');
+const cloudformationSchema = require('../../../../../lib/utils/serverless-utils/cloudformation-schema');
+
+const load = (input) => yaml.load(input, { schema: cloudformationSchema });
+
+describe('serverless-utils/cloudformation-schema', () => {
+  it('should keep date-shaped plain scalars and mapping keys as strings', () => {
+    expect(load('date: 2012-10-17').date).to.equal('2012-10-17');
+    expect(load('dateTime: 2020-12-12T00:00:00Z').dateTime).to.equal('2020-12-12T00:00:00Z');
+    expect(load('spaced: 2020-12-12 00:00:00').spaced).to.equal('2020-12-12 00:00:00');
+    expect(load('map:\n  2012-10-17: value').map).to.deep.equal({ '2012-10-17': 'value' });
+  });
+
+  it('should construct a Date for an explicit timestamp tag', () => {
+    expect(load('date: !!timestamp 2020-12-12').date).to.be.instanceOf(Date);
+  });
+});