diff --git a/src/validation/joi/joi.shared.schemas.ts b/src/validation/joi/joi.shared.schemas.ts
index d0f16a5..ac53811 100644
--- a/src/validation/joi/joi.shared.schemas.ts
+++ b/src/validation/joi/joi.shared.schemas.ts
@@ -4,7 +4,6 @@ import {
   _stringEnumKeys,
   _stringEnumValues,
   BaseDBEntity,
-  IsoDate,
   IsoDateTime,
   NumberEnum,
   StringEnum,
@@ -24,7 +23,7 @@ export const numberSchema = Joi.number()
 export const numberSchemaTyped = <T>(): NumberSchema<T> => Joi.number<T>()
 export const integerSchema = Joi.number().integer()
 export const percentageSchema = Joi.number().integer().min(0).max(100)
-export const dateStringSchema = stringSchema.dateString() as StringSchema<IsoDate>
+export const dateStringSchema = stringSchema.dateString()
 export const binarySchema = Joi.binary()
 export const dateObjectSchema = Joi.object().instance(Date)
 
diff --git a/src/validation/joi/string.extensions.test.ts b/src/validation/joi/string.extensions.test.ts
index 126de87..2f92cc7 100644
--- a/src/validation/joi/string.extensions.test.ts
+++ b/src/validation/joi/string.extensions.test.ts
@@ -1,6 +1,9 @@
+import { expectTypeOf } from '@naturalcycles/dev-lib/dist/testing'
 import { IsoDate, localTime } from '@naturalcycles/js-lib'
 import { testValidation } from '../../test/validation.test.util'
 import { stringSchema } from './joi.shared.schemas'
+import { validate } from './joi.validation.util'
+import { StringSchema } from './string.extensions'
 
 test('dateString', () => {
   const schema = stringSchema.dateString()
@@ -15,6 +18,11 @@ test('dateString', () => {
 test('dateString min/max', async () => {
   const schema = stringSchema.dateString('2017-06-21' as IsoDate, '2017-06-23' as IsoDate)
 
+  expectTypeOf(schema).toEqualTypeOf<StringSchema<IsoDate>>()
+
+  const value = validate('2017-06-22' as IsoDate, schema)
+  expectTypeOf(value).toEqualTypeOf<IsoDate>()
+
   testValidation(
     schema,
     ['2017-06-21', '2017-06-22', '2017-06-23'],
diff --git a/src/validation/joi/string.extensions.ts b/src/validation/joi/string.extensions.ts
index 4410de3..7db57f0 100644
--- a/src/validation/joi/string.extensions.ts
+++ b/src/validation/joi/string.extensions.ts
@@ -3,7 +3,7 @@ import type Joi from 'joi'
 import { Extension, StringSchema as JoiStringSchema } from 'joi'
 
 export interface StringSchema<TSchema = string> extends JoiStringSchema<TSchema> {
-  dateString: (min?: IsoDate | 'today', max?: IsoDate | 'today') => this
+  dateString: (min?: IsoDate | 'today', max?: IsoDate | 'today') => StringSchema<IsoDate>
 }
 
 export interface JoiDateStringOptions {