diff --git a/src/isValid/index.js b/src/isValid/index.ts
similarity index 85%
rename from src/isValid/index.js
rename to src/isValid/index.ts
index 854c97e378..e4dc1e2ac4 100644
--- a/src/isValid/index.js
+++ b/src/isValid/index.ts
@@ -1,3 +1,4 @@
+import isDate from '../isDate/index'
 import toDate from '../toDate/index'
 import requiredArgs from '../_lib/requiredArgs/index'
 
@@ -45,22 +46,25 @@ import requiredArgs from '../_lib/requiredArgs/index'
  *
  * @example
  * // For the valid date:
- * var result = isValid(new Date(2014, 1, 31))
+ * const result = isValid(new Date(2014, 1, 31))
  * //=> true
  *
  * @example
  * // For the value, convertable into a date:
- * var result = isValid(1393804800000)
+ * const result = isValid(1393804800000)
  * //=> true
  *
  * @example
  * // For the invalid date:
- * var result = isValid(new Date(''))
+ * const result = isValid(new Date(''))
  * //=> false
  */
-export default function isValid(dirtyDate) {
+export default function isValid(dirtyDate: unknown): boolean {
   requiredArgs(1, arguments)
 
-  var date = toDate(dirtyDate)
-  return !isNaN(date)
+  if (!isDate(dirtyDate) && typeof dirtyDate !== 'number') {
+    return false
+  }
+  const date = toDate(dirtyDate)
+  return !isNaN(Number(date))
 }
diff --git a/src/isValid/test.js b/src/isValid/test.js
deleted file mode 100644
index 95cb078214..0000000000
--- a/src/isValid/test.js
+++ /dev/null
@@ -1,31 +0,0 @@
-// @flow
-/* eslint-env mocha */
-
-import assert from 'power-assert'
-import isValid from '.'
-
-describe('isValid', function() {
-  it('returns true if the given date is valid', function() {
-    var result = isValid(new Date())
-    assert(result === true)
-  })
-
-  it('returns false if the given date is invalid', function() {
-    var result = isValid(new Date(''))
-    assert(result === false)
-  })
-
-  it('accepts a timestamp', function() {
-    assert(isValid(new Date(2014, 1 /* Feb */, 11).getTime()) === true)
-    assert(isValid(NaN) === false)
-  })
-
-  it('treats null as an invalid date', function() {
-    var result = isValid(null)
-    assert(result === false)
-  })
-
-  it('throws TypeError exception if passed less than 1 argument', function() {
-    assert.throws(isValid.bind(null), TypeError)
-  })
-})
diff --git a/src/isValid/test.ts b/src/isValid/test.ts
new file mode 100644
index 0000000000..b24210fbdf
--- /dev/null
+++ b/src/isValid/test.ts
@@ -0,0 +1,31 @@
+/* eslint-env mocha */
+
+import assert from 'assert'
+import isValid from '.'
+
+describe('isValid', function () {
+  it('returns true if the given date is valid', function () {
+    const result = isValid(new Date())
+    assert(result === true)
+  })
+
+  it('returns false if the given date is invalid', function () {
+    const result = isValid(new Date(''))
+    assert(result === false)
+  })
+
+  it('accepts a timestamp', function () {
+    assert(isValid(new Date(2014, 1 /* Feb */, 11).getTime()) === true)
+    assert(isValid(NaN) === false)
+  })
+
+  it('treats null as an invalid date', function () {
+    const result = isValid(null)
+    assert(result === false)
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', function () {
+    // @ts-expect-error
+    assert.throws(isValid.bind(null), TypeError)
+  })
+})