diff --git a/packages/time/src/formatter/buildDateFormatter.ts b/packages/time/src/formatter/buildDateFormatter.ts
index 680fba2..b552078 100644
--- a/packages/time/src/formatter/buildDateFormatter.ts
+++ b/packages/time/src/formatter/buildDateFormatter.ts
@@ -1,9 +1,9 @@
 import { getDefaultLocale } from "../utils/dateDefaults";
 import { extractLocaleOptions } from "./extractLocaleOptions";
-import type { IDateFormatterBuildParams } from "./shared";
+import type { DateFormatterBuildParams } from "./shared";
 
 /**
- * @typedef {Object} IDateFormatterOptions
+ * @typedef {Object} DateFormatterOptions
  * @property {string} [localeMatcher]
  * @property {string} [calendar]
  * @property {string} [numberingSystem]
@@ -20,9 +20,9 @@ import type { IDateFormatterBuildParams } from "./shared";
  * 
  * @typedef {string | Intl.Locale | string[] | Intl.Locale[]} Locale
  * 
- * @typedef {Object} IDateFormatterBuildParams
+ * @typedef {Object} DateFormatterBuildParams
  * @property {Locale} [locale]
- * @property {string | IDateFormatterOptions} [options]
+ * @property {string | DateFormatterOptions} [options]
  */
 
 /**
@@ -43,13 +43,13 @@ import type { IDateFormatterBuildParams } from "./shared";
  * When using UTC date strings, it is suggested that you use the 'options' object
  * to set the 'timeZone' when building the formatter. The 'timeZone' is defaulted
  * to the user's browser timezone.
- * @param {IDateFormatterBuildParams} [param0]
+ * @param {DateFormatterBuildParams} [param0]
  * @returns Intl.DateTimeFormat
  */
 export function buildDateFormatter({
   locale = getDefaultLocale(), 
   options
-}: IDateFormatterBuildParams = {}): Intl.DateTimeFormat {
+}: DateFormatterBuildParams = {}): Intl.DateTimeFormat {
   const opts = (typeof options === 'string') ? { dateStyle: options } : options ?? {};
   const {formatOptions = {}, ...localeOptions} = extractLocaleOptions(opts);
   const { dateStyle, ...rest } = formatOptions;
diff --git a/packages/time/src/formatter/buildDateTimeFormatter.ts b/packages/time/src/formatter/buildDateTimeFormatter.ts
index bc02c82..b96c625 100644
--- a/packages/time/src/formatter/buildDateTimeFormatter.ts
+++ b/packages/time/src/formatter/buildDateTimeFormatter.ts
@@ -1,9 +1,9 @@
 import { getDefaultLocale } from "../utils/dateDefaults";
 import { extractLocaleOptions } from "./extractLocaleOptions";
-import type { IDateTimeFormatterBuildParams } from "./shared";
+import type { DateTimeFormatterBuildParams } from "./shared";
 
 /**
- * @typedef {Object} IDateTimeFormatterOptions
+ * @typedef {Object} DateTimeFormatterOptions
  * @property {string} [localeMatcher]
  * @property {string} [calendar]
  * @property {string} [numberingSystem]
@@ -27,9 +27,9 @@ import type { IDateTimeFormatterBuildParams } from "./shared";
  * 
  * @typedef {string | Intl.Locale | string[] | Intl.Locale[]} Locale
  * 
- * @typedef {Object} IDateTimeFormatterOptions
+ * @typedef {Object} DateTimeFormatterOptions
  * @property {Locale} [locale]
- * @property {string | IDateFormatterOptions} [options]
+ * @property {string | DateFormatterOptions} [options]
  */
 
 /**
@@ -56,7 +56,7 @@ import type { IDateTimeFormatterBuildParams } from "./shared";
 export function buildDateTimeFormatter({
   locale = getDefaultLocale(), 
   options
-}: IDateTimeFormatterBuildParams): Intl.DateTimeFormat {
+}: DateTimeFormatterBuildParams): Intl.DateTimeFormat {
   const opts = (typeof options === 'string') ? { dateStyle: options, timeStyle: options } : options ?? {};
   const {formatOptions = {}, ...localeOptions} = extractLocaleOptions(opts);
   const { dateStyle, timeStyle, ...rest } = formatOptions;
diff --git a/packages/time/src/formatter/buildTimeFormatter.ts b/packages/time/src/formatter/buildTimeFormatter.ts
index 9ddee8d..3ea3a4c 100644
--- a/packages/time/src/formatter/buildTimeFormatter.ts
+++ b/packages/time/src/formatter/buildTimeFormatter.ts
@@ -1,9 +1,9 @@
 import { getDefaultLocale } from "../utils/dateDefaults";
 import { extractLocaleOptions } from "./extractLocaleOptions";
-import type { ITimeFormatterBuildParams } from "./shared";
+import type { TimeFormatterBuildParams } from "./shared";
 
 /**
- * @typedef {Object} ITimeFormatterOptions
+ * @typedef {Object} TimeFormatterOptions
  * @property {string} [localeMatcher]
  * @property {string} [calendar]
  * @property {string} [numberingSystem]
@@ -21,9 +21,9 @@ import type { ITimeFormatterBuildParams } from "./shared";
  * 
  * @typedef {string | Intl.Locale | string[] | Intl.Locale[]} Locale
  * 
- * @typedef {Object} ITimeFormatterBuildParams
+ * @typedef {Object} TimeFormatterBuildParams
  * @property {Locale} [locale]
- * @property {string | IDateFormatterOptions} [options]
+ * @property {string | DateFormatterOptions} [options]
  */
 
 /**
@@ -44,13 +44,13 @@ import type { ITimeFormatterBuildParams } from "./shared";
  * When using UTC date strings, it is suggested that you use the 'options' object
  * to set the 'timeZone' when building the formatter. The 'timeZone' is defaulted
  * to the user's browser timezone.
- * @param {IDateFormatterBuildParams} [param0]
+ * @param {DateFormatterBuildParams} [param0]
  * @returns Intl.DateTimeFormat
  */
 export function buildTimeFormatter({
   locale = getDefaultLocale(), 
   options
-}: ITimeFormatterBuildParams): Intl.DateTimeFormat {
+}: TimeFormatterBuildParams): Intl.DateTimeFormat {
   const opts = (typeof options === 'string') ? { dateStyle: options } : options ?? {};
   const {formatOptions = {}, ...localeOptions} = extractLocaleOptions(opts);
   const { timeStyle, ...rest } = formatOptions;
@@ -59,4 +59,5 @@ export function buildTimeFormatter({
     ...(timeStyle ? { timeStyle } : rest),
   };
   return new Intl.DateTimeFormat(locale, newOptions);
-}
\ No newline at end of file
+}
+
diff --git a/packages/time/src/formatter/extractLocaleOptions.ts b/packages/time/src/formatter/extractLocaleOptions.ts
index c7fed83..be50e99 100644
--- a/packages/time/src/formatter/extractLocaleOptions.ts
+++ b/packages/time/src/formatter/extractLocaleOptions.ts
@@ -1,8 +1,13 @@
-import { getDefaultCalendar, getDefaultTimeZone } from "../utils/dateDefaults";
-import type { IDateFormatterOptions, IDateTimeFormatterOptions, ITimeFormatterOptions  } from "./shared";
+import { getDefaultCalendar, getDefaultTimeZone } from '../utils/dateDefaults'
+import type {
+  DateFormatterOptions,
+  DateTimeFormatterOptions,
+  StrictUnion,
+  TimeFormatterOptions,
+} from './shared'
 
 /**
- * @typedef {Object} IDateFormatterOptions
+ * @typedef {Object} DateFormatterOptions
  * @property {string} [localeMatcher]
  * @property {string} [calendar]
  * @property {string} [numberingSystem]
@@ -16,8 +21,8 @@ import type { IDateFormatterOptions, IDateTimeFormatterOptions, ITimeFormatterOp
  * @property {string} [month]
  * @property {string} [day]
  * @property {string} [dateStyle]
- * 
- * @typedef {Object} ITimeFormatterOptions
+ *
+ * @typedef {Object} TimeFormatterOptions
  * @property {string} [localeMatcher]
  * @property {string} [calendar]
  * @property {string} [numberingSystem]
@@ -32,8 +37,8 @@ import type { IDateFormatterOptions, IDateTimeFormatterOptions, ITimeFormatterOp
  * @property {number} [fractionalSecondDigits]
  * @property {string} [timeZoneName]
  * @property {string} [timeStyle]
- * 
- * @typedef {Object} IDateTimeFormatterOptions
+ *
+ * @typedef {Object} DateTimeFormatterOptions
  * @property {string} [localeMatcher]
  * @property {string} [calendar]
  * @property {string} [numberingSystem]
@@ -56,26 +61,32 @@ import type { IDateFormatterOptions, IDateTimeFormatterOptions, ITimeFormatterOp
  * @property {string} [timeStyle]
  */
 
- /** 
+/**
  * Function: extractLocaleOptions
  * This function is used to extract the locale options from the 'options' parameter of the various
  * formatter 'build' functions.
- * 
+ *
  * If 'calender' or 'timeZone' are not provided, the default values provided by the
  * Intl.DateTimeFormat().resolvedOptions() are used.
- * @param {IDateFormatterOptions | IDateTimeFormatterOptions | ITimeFormatterOptions} param0 
- * @returns 
+ * @param {DateFormatterOptions | DateTimeFormatterOptions | TimeFormatterOptions} param0
+ * @returns
  */
 export function extractLocaleOptions({
-  localeMatcher, 
-  calendar = getDefaultCalendar(), 
-  numberingSystem, 
-  hour12, 
-  hourCycle, 
-  timeZone = getDefaultTimeZone(), 
+  localeMatcher,
+  calendar = getDefaultCalendar(),
+  numberingSystem,
+  hour12,
+  hourCycle,
+  timeZone = getDefaultTimeZone(),
   ...formatOptions
-}: IDateFormatterOptions | IDateTimeFormatterOptions | ITimeFormatterOptions) {
+}: StrictUnion<DateFormatterOptions | DateTimeFormatterOptions | TimeFormatterOptions>) {
   return {
-    localeMatcher, calendar, numberingSystem, hour12, hourCycle, timeZone, formatOptions
-  };
-}
\ No newline at end of file
+    localeMatcher,
+    calendar,
+    numberingSystem,
+    hour12,
+    hourCycle,
+    timeZone,
+    formatOptions,
+  }
+}
diff --git a/packages/time/src/formatter/shared.ts b/packages/time/src/formatter/shared.ts
index af409b9..8bbe07e 100644
--- a/packages/time/src/formatter/shared.ts
+++ b/packages/time/src/formatter/shared.ts
@@ -1,4 +1,4 @@
-export interface ILocaleFormatterOptions {
+export interface LocaleFormatterOptions {
   localeMatcher?: 'lookup' | 'best fit';
   calendar?: string;
   numberingSystem?: string;
@@ -9,7 +9,7 @@ export interface ILocaleFormatterOptions {
 
 type FormatStyle = 'full' | 'long' | 'medium' | 'short';
 
-export interface IDateFormatterOptions extends ILocaleFormatterOptions {
+export interface DateFormatterOptions extends LocaleFormatterOptions {
   formatMatcher?: 'basic' | 'best fit';
   weekday?: 'narrow' | 'short' | 'long';
   era?: 'narrow' | 'short' | 'long';
@@ -20,7 +20,7 @@ export interface IDateFormatterOptions extends ILocaleFormatterOptions {
   dateStyle?: FormatStyle;
 }
 
-export interface ITimeFormatterOptions extends ILocaleFormatterOptions {
+export interface TimeFormatterOptions extends LocaleFormatterOptions {
   formatMatcher?: 'basic' | 'best fit';
   dayPeriod?: 'narrow' | 'short' | 'long';
   hour?: '2-digit' | 'numeric';
@@ -32,20 +32,17 @@ export interface ITimeFormatterOptions extends ILocaleFormatterOptions {
   timeStyle?: FormatStyle;
 }
 
-export interface IDateTimeFormatterOptions extends IDateFormatterOptions, ITimeFormatterOptions {
-}
+export type DateTimeFormatterOptions = DateFormatterOptions & TimeFormatterOptions
 
-export interface IDateFormatterBuildParams {
-  locale?: string | Intl.Locale | Array<string|Intl.Locale>;
-  options?: FormatStyle | IDateFormatterOptions;
+interface FormatterBuildParams<TOptions extends LocaleFormatterOptions> {
+  locale?: string | Intl.Locale | Array<string | Intl.Locale>;
+  options?: FormatStyle | TOptions;
 }
 
-export interface ITimeFormatterBuildParams {
-  locale?: string | Intl.Locale | Array<string|Intl.Locale>;
-  options?: FormatStyle | ITimeFormatterOptions;
-}
+export type DateFormatterBuildParams = FormatterBuildParams<DateFormatterOptions>;
+export type TimeFormatterBuildParams = FormatterBuildParams<TimeFormatterOptions>;
+export type DateTimeFormatterBuildParams = FormatterBuildParams<DateTimeFormatterOptions>;
 
-export interface IDateTimeFormatterBuildParams {
-  locale?: string | Intl.Locale | Array<string|Intl.Locale>;
-  options?: FormatStyle | IDateTimeFormatterOptions;
-}
\ No newline at end of file
+export type UnionKeys<T> = T extends T ? keyof T : never;
+export type StrictUnionHelper<T, TAll> = T extends any ? T & Partial<Record<Exclude<UnionKeys<TAll>, keyof T>, never>> : never;
+export type StrictUnion<T> = StrictUnionHelper<T, T>
\ No newline at end of file
diff --git a/packages/time/src/tests/isValidDate.test.ts b/packages/time/src/tests/isValidDate.test.ts
index 57a3556..4d01970 100644
--- a/packages/time/src/tests/isValidDate.test.ts
+++ b/packages/time/src/tests/isValidDate.test.ts
@@ -1,16 +1,34 @@
-import {describe, expect, test} from 'vitest';
-import {isValidDate} from '../utils/isValidDate';
+import { describe, expect, test } from 'vitest'
+import { isValidDate } from '../utils/isValidDate'
 
 describe('isValidDate', () => {
   test('should return true for a valid date', () => {
-    expect(isValidDate(new Date())).toBe(true);
-  });
+    const date = new Date();
+    expect(isValidDate(date)).toBe(true)
+  })
 
-  test('should return false for an invalid date', () => {
-    expect(isValidDate(new Date("invalid"))).toBe(false);
-  });
+  test.each([
+    '2021-10-10',
+    new Date('invalid'),
+    {},
+    undefined,
+    null,
+    NaN,
+    0,
+  ])('should return false for invalid date %p', (date) => {
+    expect(isValidDate(date)).toBe(false)
+  })
 
-  test("should return false for null", () => {
-    expect(isValidDate(null)).toBe(false);
-  });
-});
\ No newline at end of file
+  test('should assert type guards correctly', () => {
+    const notADate = 'not a date';
+    if (isValidDate(notADate)) {
+      expect(notADate).toBeInstanceOf(Date)
+      notADate.getDate()
+    } else {
+      expect(() => {
+        // @ts-expect-error
+        notADate.getTime()
+      }).toThrowError()
+    }
+  })
+})
diff --git a/packages/time/src/utils/isValidDate.ts b/packages/time/src/utils/isValidDate.ts
index 987df31..c0d1599 100644
--- a/packages/time/src/utils/isValidDate.ts
+++ b/packages/time/src/utils/isValidDate.ts
@@ -4,9 +4,6 @@
  * @param date Date
  * @returns boolean
  */
-export function isValidDate(date: any): boolean {
-  if (Object.prototype.toString.call(date) !== '[object Date]') {
-    return false;
-  }
-  return date.getTime() === date.getTime();
-}
\ No newline at end of file
+export function isValidDate(date: unknown): date is Date {
+  return date instanceof Date && !isNaN(date.getTime());
+}
diff --git a/packages/time/src/utils/validateDate.ts b/packages/time/src/utils/validateDate.ts
index 9416553..eb7556f 100644
--- a/packages/time/src/utils/validateDate.ts
+++ b/packages/time/src/utils/validateDate.ts
@@ -19,5 +19,5 @@ export function validateDate({date, parse = parser, errorMessage = `Invalid Date
   if (!isValidDate(d)) {
     throw new Error(`${errorMessage}: "${date}"`);
   }
-  return d as Date;
+  return d;
 }
\ No newline at end of file