From 4e73b176459c40972f6eea53c850f614c1bc60c3 Mon Sep 17 00:00:00 2001
From: Pawel Stanecki <pawel.stanecki@sandstream.pl>
Date: Thu, 12 Dec 2019 15:49:55 +0100
Subject: [PATCH 1/6] Update dateDiff.ts and add tests

---
 README.md             |  8 ++++++++
 async/README.md       |  6 ++++++
 async/debounce.md     |  2 ++
 async/delay.md        |  2 ++
 async/sequence.js     | 12 +++++-------
 async/sequence.md     |  2 ++
 date/README.md        |  2 ++
 date/dateDiff.json    |  2 +-
 date/dateDiff.md      |  2 ++
 date/dateDiff.test.ts | 18 ++++++++++++++++--
 date/dateDiff.ts      |  2 +-
 11 files changed, 47 insertions(+), 11 deletions(-)

diff --git a/README.md b/README.md
index b4e2b621..371cbafd 100644
--- a/README.md
+++ b/README.md
@@ -198,10 +198,16 @@ Zips given arrays together with the given function.
 
 #### debounce
 
+Makes the function run after the given period of not being called. Useful to delay input submission for autocomplete etc.
+
 #### delay
 
+When awaited, delays the execution by the given number of milliseconds.
+
 #### sequence
 
+Runs the given tasks in a sequence.
+
 ### date
 
 #### byDateWithFallback
@@ -210,6 +216,8 @@ Zips given arrays together with the given function.
 
 #### dateDiff
 
+Calculates the difference in milliseconds between two dates.
+
 #### dateInRange
 
 #### dayRange
diff --git a/async/README.md b/async/README.md
index 0ea12fa9..e2381449 100644
--- a/async/README.md
+++ b/async/README.md
@@ -1,5 +1,11 @@
 # debounce
 
+Makes the function run after the given period of not being called. Useful to delay input submission for autocomplete etc.
+
 # delay
 
+When awaited, delays the execution by the given number of milliseconds.
+
 # sequence
+
+Runs the given tasks in a sequence.
diff --git a/async/debounce.md b/async/debounce.md
index dce82919..a34e4c1e 100644
--- a/async/debounce.md
+++ b/async/debounce.md
@@ -1 +1,3 @@
 # debounce
+
+Makes the function run after the given period of not being called. Useful to delay input submission for autocomplete etc.
diff --git a/async/delay.md b/async/delay.md
index 021604ae..da6fc617 100644
--- a/async/delay.md
+++ b/async/delay.md
@@ -1 +1,3 @@
 # delay
+
+When awaited, delays the execution by the given number of milliseconds.
diff --git a/async/sequence.js b/async/sequence.js
index be0ac9e2..09738849 100644
--- a/async/sequence.js
+++ b/async/sequence.js
@@ -1,13 +1,11 @@
 export default async tasks => {
   const results = tasks.map(() => undefined);
-  await tasks.reduce((chain, current, i) => {
-    return chain.then(() =>
-      current().then(x => {
-        results[i] = x;
+  await tasks.reduce(async (chain, current, i) => {
+    await chain;
+    const x = await current();
+    results[i] = x;
 
-        return x;
-      })
-    );
+    return x;
   }, Promise.resolve());
 
   return results;
diff --git a/async/sequence.md b/async/sequence.md
index a6fb8760..32faf064 100644
--- a/async/sequence.md
+++ b/async/sequence.md
@@ -1 +1,3 @@
 # sequence
+
+Runs the given tasks in a sequence.
diff --git a/date/README.md b/date/README.md
index 8c1e867e..5dd42f97 100644
--- a/date/README.md
+++ b/date/README.md
@@ -4,6 +4,8 @@
 
 # dateDiff
 
+Calculates the difference in milliseconds between two dates.
+
 # dateInRange
 
 # dayRange
diff --git a/date/dateDiff.json b/date/dateDiff.json
index c50775e0..b449bd97 100644
--- a/date/dateDiff.json
+++ b/date/dateDiff.json
@@ -1,6 +1,6 @@
 {
   "name": "dateDiff",
-  "description": "TODO: Fill short description here.",
+  "description": "Calculates the difference in milliseconds between two dates.",
   "signature": "TODO: Fill type signature here.",
   "examples": [
     {
diff --git a/date/dateDiff.md b/date/dateDiff.md
index a299ea56..e74d2aea 100644
--- a/date/dateDiff.md
+++ b/date/dateDiff.md
@@ -1 +1,3 @@
 # dateDiff
+
+Calculates the difference in milliseconds between two dates.
diff --git a/date/dateDiff.test.ts b/date/dateDiff.test.ts
index 7fd8c8bb..a650200e 100644
--- a/date/dateDiff.test.ts
+++ b/date/dateDiff.test.ts
@@ -3,7 +3,21 @@
 import dateDiff from "./dateDiff.ts";
 
 describe("dateDiff", () => {
-  it.skip("TODO", () => {
-    expect(dateDiff()).toBeDefined();
+  it("returns the difference of 24 hours in miliseconds", () => {
+    const firstDate = new Date("2019-12-18T12:00:00");
+    const secondDate = new Date("2019-12-17T12:00:00");
+
+    const dayInMiliseconds = 86400000;
+
+    expect(dateDiff(firstDate, secondDate)).toEqual(dayInMiliseconds);
+  });
+
+  it("returns 0 difference of eqact same dates", () => {
+    const firstDate = new Date("2019-10-10T10:00:00");
+    const secondDate = new Date("2019-10-10T10:00:00");
+
+    const noDifference = 0;
+
+    expect(dateDiff(firstDate, secondDate)).toEqual(noDifference);
   });
 });
diff --git a/date/dateDiff.ts b/date/dateDiff.ts
index 2e4d2ce6..aadcb67b 100644
--- a/date/dateDiff.ts
+++ b/date/dateDiff.ts
@@ -1,4 +1,4 @@
-export default (a, b) => {
+export default (a: string | Date, b: string | Date): number => {
   const d1 = new Date(a);
   const d2 = new Date(b);
 

From e904ce3c112d84149db30d757eba1145c154a6a6 Mon Sep 17 00:00:00 2001
From: Pawel Stanecki <pawel.stanecki@sandstream.pl>
Date: Thu, 12 Dec 2019 16:06:47 +0100
Subject: [PATCH 2/6] Add types in dateInRange.ts

---
 date/dateInRange.ts | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/date/dateInRange.ts b/date/dateInRange.ts
index 29997ef7..cfed7428 100644
--- a/date/dateInRange.ts
+++ b/date/dateInRange.ts
@@ -1,4 +1,6 @@
-export default (from, to) => (date = new Date()) => {
+export default (from: string | Date, to: string | Date) => (
+  date = new Date()
+): boolean => {
   const dateTime = new Date(date).getTime();
   const fromTime = new Date(from).getTime();
   const toTime = new Date(to).getTime();

From f941666560edc8124ca03aa7d897d7805dcc80ea Mon Sep 17 00:00:00 2001
From: Pawel Stanecki <pawel.stanecki@sandstream.pl>
Date: Fri, 13 Dec 2019 09:11:19 +0100
Subject: [PATCH 3/6] Updated dateInRange

---
 README.md                |  6 ++++++
 date/README.md           |  6 ++++++
 date/dateInRange.json    |  2 +-
 date/dateInRange.md      |  6 ++++++
 date/dateInRange.test.ts | 20 ++++++++++++++++++--
 5 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index 371cbafd..e5982300 100644
--- a/README.md
+++ b/README.md
@@ -220,6 +220,12 @@ Calculates the difference in milliseconds between two dates.
 
 #### dateInRange
 
+##### Type signature
+
+```
+Checks if provided date is within the range.
+```
+
 #### dayRange
 
 #### daysInMonths
diff --git a/date/README.md b/date/README.md
index 5dd42f97..f3d1edec 100644
--- a/date/README.md
+++ b/date/README.md
@@ -8,6 +8,12 @@ Calculates the difference in milliseconds between two dates.
 
 # dateInRange
 
+## Type signature
+
+```
+Checks if provided date is within the range.
+```
+
 # dayRange
 
 # daysInMonths
diff --git a/date/dateInRange.json b/date/dateInRange.json
index 35f60ade..dce8b0e7 100644
--- a/date/dateInRange.json
+++ b/date/dateInRange.json
@@ -1,7 +1,7 @@
 {
   "name": "dateInRange",
   "description": "TODO: Fill short description here.",
-  "signature": "TODO: Fill type signature here.",
+  "signature": "Checks if provided date is within the range.",
   "examples": [
     {
       "language": "javascript",
diff --git a/date/dateInRange.md b/date/dateInRange.md
index 87f3e12c..0aeb1578 100644
--- a/date/dateInRange.md
+++ b/date/dateInRange.md
@@ -1 +1,7 @@
 # dateInRange
+
+## Type signature
+
+```
+Checks if provided date is within the range.
+```
diff --git a/date/dateInRange.test.ts b/date/dateInRange.test.ts
index 0ba4939f..96ceba00 100644
--- a/date/dateInRange.test.ts
+++ b/date/dateInRange.test.ts
@@ -3,7 +3,23 @@
 import dateInRange from "./dateInRange.ts";
 
 describe("dateInRange", () => {
-  it.skip("TODO", () => {
-    expect(dateInRange()).toBeDefined();
+  it("provided date is within the day range", () => {
+    const startRangeDate = new Date("2019-05-28T12:00:00");
+    const endRangeDate = new Date("2019-05-29T12:00:00");
+    const dateToCheck = new Date("2019-05-28T24:00:00");
+
+    expect(dateInRange(startRangeDate, endRangeDate)(dateToCheck)).toEqual(
+      true
+    );
+  });
+
+  it("provided date is not within the year range", () => {
+    const startRangeDate = new Date("2018-05-28T12:00:00");
+    const endRangeDate = new Date("2019-05-29T12:00:00");
+    const dateToCheck = new Date("2017-05-28T24:00:00");
+
+    expect(dateInRange(startRangeDate, endRangeDate)(dateToCheck)).toEqual(
+      false
+    );
   });
 });

From 312c53ef897f6d016d7a89b9f0fc03f963bc6a7a Mon Sep 17 00:00:00 2001
From: Pawel Stanecki <pawel.stanecki@sandstream.pl>
Date: Fri, 13 Dec 2019 09:41:10 +0100
Subject: [PATCH 4/6] Update datesInMonths

---
 date/dateInRange.json     |  4 ++--
 date/daysInMonths.json    |  2 +-
 date/daysInMonths.test.ts | 38 ++++++++++++++++++++++++++++++++++++--
 date/daysInMonths.ts      |  2 +-
 4 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/date/dateInRange.json b/date/dateInRange.json
index dce8b0e7..bf5236e3 100644
--- a/date/dateInRange.json
+++ b/date/dateInRange.json
@@ -1,7 +1,7 @@
 {
   "name": "dateInRange",
-  "description": "TODO: Fill short description here.",
-  "signature": "Checks if provided date is within the range.",
+  "description": "Checks if provided date is within the range.",
+  "signature": "TODO: Fill type signature here.",
   "examples": [
     {
       "language": "javascript",
diff --git a/date/daysInMonths.json b/date/daysInMonths.json
index 054f2c19..bb1402f2 100644
--- a/date/daysInMonths.json
+++ b/date/daysInMonths.json
@@ -1,6 +1,6 @@
 {
   "name": "daysInMonths",
-  "description": "TODO: Fill short description here.",
+  "description": "Returns numbers of days in all months of the year having regard to leap year.",
   "signature": "TODO: Fill type signature here.",
   "examples": [
     {
diff --git a/date/daysInMonths.test.ts b/date/daysInMonths.test.ts
index bed95fea..c50a523a 100644
--- a/date/daysInMonths.test.ts
+++ b/date/daysInMonths.test.ts
@@ -3,7 +3,41 @@
 import daysInMonths from "./daysInMonths.ts";
 
 describe("daysInMonths", () => {
-  it.skip("TODO", () => {
-    expect(daysInMonths()).toBeDefined();
+  it("returns number of days in a months in a leap year", () => {
+    const leapYear = true;
+    const numberOfDaysInMonthsInLeapYear = [
+      31,
+      29,
+      31,
+      30,
+      31,
+      30,
+      31,
+      31,
+      30,
+      31,
+      30,
+      31
+    ];
+    expect(daysInMonths(leapYear)).toEqual(numberOfDaysInMonthsInLeapYear);
+  });
+
+  it("returns number of days in a months in a year", () => {
+    const leapYear = false;
+    const numberOfDaysInMonths = [
+      31,
+      28,
+      31,
+      30,
+      31,
+      30,
+      31,
+      31,
+      30,
+      31,
+      30,
+      31
+    ];
+    expect(daysInMonths(leapYear)).toEqual(numberOfDaysInMonths);
   });
 });
diff --git a/date/daysInMonths.ts b/date/daysInMonths.ts
index 2ad5657f..672f2333 100644
--- a/date/daysInMonths.ts
+++ b/date/daysInMonths.ts
@@ -1,4 +1,4 @@
-export default leapYear => [
+export default (leapYear): Number[] => [
   31,
   leapYear ? 29 : 28,
   31,

From 30e4006b3683e1dc08defea681bcbce5961b096f Mon Sep 17 00:00:00 2001
From: Pawel Stanecki <pawel.stanecki@sandstream.pl>
Date: Fri, 13 Dec 2019 09:56:26 +0100
Subject: [PATCH 5/6] build and test daysinMonths

---
 README.md                 | 6 ++----
 date/README.md            | 6 ++----
 date/dateInRange.md       | 4 ----
 date/daysInMonths.md      | 2 ++
 date/daysInMonths.test.ts | 4 ++++
 date/daysInMonths.ts      | 2 +-
 6 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/README.md b/README.md
index e5982300..8b0fd19f 100644
--- a/README.md
+++ b/README.md
@@ -220,16 +220,14 @@ Calculates the difference in milliseconds between two dates.
 
 #### dateInRange
 
-##### Type signature
-
-```
 Checks if provided date is within the range.
-```
 
 #### dayRange
 
 #### daysInMonths
 
+Returns numbers of days in all months of the year having regard to leap year.
+
 #### daysInYear
 
 #### displayMonth
diff --git a/date/README.md b/date/README.md
index f3d1edec..6514415d 100644
--- a/date/README.md
+++ b/date/README.md
@@ -8,16 +8,14 @@ Calculates the difference in milliseconds between two dates.
 
 # dateInRange
 
-## Type signature
-
-```
 Checks if provided date is within the range.
-```
 
 # dayRange
 
 # daysInMonths
 
+Returns numbers of days in all months of the year having regard to leap year.
+
 # daysInYear
 
 # displayMonth
diff --git a/date/dateInRange.md b/date/dateInRange.md
index 0aeb1578..226f3cc7 100644
--- a/date/dateInRange.md
+++ b/date/dateInRange.md
@@ -1,7 +1,3 @@
 # dateInRange
 
-## Type signature
-
-```
 Checks if provided date is within the range.
-```
diff --git a/date/daysInMonths.md b/date/daysInMonths.md
index 6257a25f..d91d7b5a 100644
--- a/date/daysInMonths.md
+++ b/date/daysInMonths.md
@@ -1 +1,3 @@
 # daysInMonths
+
+Returns numbers of days in all months of the year having regard to leap year.
diff --git a/date/daysInMonths.test.ts b/date/daysInMonths.test.ts
index c50a523a..3bd70300 100644
--- a/date/daysInMonths.test.ts
+++ b/date/daysInMonths.test.ts
@@ -5,6 +5,7 @@ import daysInMonths from "./daysInMonths.ts";
 describe("daysInMonths", () => {
   it("returns number of days in a months in a leap year", () => {
     const leapYear = true;
+
     const numberOfDaysInMonthsInLeapYear = [
       31,
       29,
@@ -19,11 +20,13 @@ describe("daysInMonths", () => {
       30,
       31
     ];
+
     expect(daysInMonths(leapYear)).toEqual(numberOfDaysInMonthsInLeapYear);
   });
 
   it("returns number of days in a months in a year", () => {
     const leapYear = false;
+
     const numberOfDaysInMonths = [
       31,
       28,
@@ -38,6 +41,7 @@ describe("daysInMonths", () => {
       30,
       31
     ];
+
     expect(daysInMonths(leapYear)).toEqual(numberOfDaysInMonths);
   });
 });
diff --git a/date/daysInMonths.ts b/date/daysInMonths.ts
index 672f2333..6a6b17b7 100644
--- a/date/daysInMonths.ts
+++ b/date/daysInMonths.ts
@@ -1,4 +1,4 @@
-export default (leapYear): Number[] => [
+export default (leapYear): number[] => [
   31,
   leapYear ? 29 : 28,
   31,

From b2b47591949ef1a0ea433362544ca92c52a69516 Mon Sep 17 00:00:00 2001
From: Pawel Stanecki <pawel.stanecki@sandstream.pl>
Date: Fri, 13 Dec 2019 11:08:52 +0100
Subject: [PATCH 6/6] Added monthnames, start/end of a day tests

---
 README.md               |  6 ++++++
 date/README.md          |  6 ++++++
 date/endOfDay.json      |  2 +-
 date/endOfDay.md        |  2 ++
 date/endOfDay.test.ts   | 19 +++++++++++++++++--
 date/endOfDay.ts        |  6 +++++-
 date/monthNames.js      |  2 +-
 date/monthNames.json    |  2 +-
 date/monthNames.md      |  2 ++
 date/monthNames.test.ts | 19 +++++++++++++++++--
 date/monthNames.ts      |  2 +-
 date/startOfDay.json    |  2 +-
 date/startOfDay.md      |  2 ++
 date/startOfDay.test.ts | 19 +++++++++++++++++--
 date/startOfDay.ts      |  6 +++++-
 15 files changed, 84 insertions(+), 13 deletions(-)

diff --git a/README.md b/README.md
index 8b0fd19f..39a4d41a 100644
--- a/README.md
+++ b/README.md
@@ -236,6 +236,8 @@ Returns numbers of days in all months of the year having regard to leap year.
 
 #### endOfDay
 
+Calculates end of a day in provided date.
+
 #### formatDate
 
 #### formatDateTime
@@ -260,6 +262,8 @@ Returns numbers of days in all months of the year having regard to leap year.
 
 #### monthNames
 
+Returns all months names.
+
 #### offsetByBit
 
 #### parseHourMinutePair
@@ -268,6 +272,8 @@ Returns numbers of days in all months of the year having regard to leap year.
 
 #### startOfDay
 
+Calculates start of a day in provided date.
+
 #### subtractDays
 
 #### toDate
diff --git a/date/README.md b/date/README.md
index 6514415d..08b92a1a 100644
--- a/date/README.md
+++ b/date/README.md
@@ -24,6 +24,8 @@ Returns numbers of days in all months of the year having regard to leap year.
 
 # endOfDay
 
+Calculates end of a day in provided date.
+
 # formatDate
 
 # formatDateTime
@@ -48,6 +50,8 @@ Returns numbers of days in all months of the year having regard to leap year.
 
 # monthNames
 
+Returns all months names.
+
 # offsetByBit
 
 # parseHourMinutePair
@@ -56,6 +60,8 @@ Returns numbers of days in all months of the year having regard to leap year.
 
 # startOfDay
 
+Calculates start of a day in provided date.
+
 # subtractDays
 
 # toDate
diff --git a/date/endOfDay.json b/date/endOfDay.json
index 3676c1c5..fbca114d 100644
--- a/date/endOfDay.json
+++ b/date/endOfDay.json
@@ -1,6 +1,6 @@
 {
   "name": "endOfDay",
-  "description": "TODO: Fill short description here.",
+  "description": "Calculates end of a day in provided date.",
   "signature": "TODO: Fill type signature here.",
   "examples": [
     {
diff --git a/date/endOfDay.md b/date/endOfDay.md
index e16e7108..878cd7ef 100644
--- a/date/endOfDay.md
+++ b/date/endOfDay.md
@@ -1 +1,3 @@
 # endOfDay
+
+Calculates end of a day in provided date.
diff --git a/date/endOfDay.test.ts b/date/endOfDay.test.ts
index d0fe743d..2e7ba82b 100644
--- a/date/endOfDay.test.ts
+++ b/date/endOfDay.test.ts
@@ -3,7 +3,22 @@
 import endOfDay from "./endOfDay.ts";
 
 describe("endOfDay", () => {
-  it.skip("TODO", () => {
-    expect(endOfDay()).toBeDefined();
+  const date = new Date("2018-12-31T13:54:33.232Z");
+
+  it("should handle timezone offsets", () => {
+    const localOffset = -180;
+
+    const end = endOfDay(date, localOffset);
+    const expectedUTC = new Date("2019-01-01T03:00:00.000Z");
+    const expectedLocal = new Date("2018-12-31T24:00:00.000-03:00");
+
+    expect(end).toEqual(expectedUTC);
+    expect(end).toEqual(expectedLocal);
+  });
+
+  it("should handle default local date time display", () => {
+    const localOffset = 0;
+
+    expect(endOfDay(date)).toEqual(endOfDay(date, localOffset));
   });
 });
diff --git a/date/endOfDay.ts b/date/endOfDay.ts
index 71f92a0f..54005c08 100644
--- a/date/endOfDay.ts
+++ b/date/endOfDay.ts
@@ -1,6 +1,10 @@
 import toLocalDateTime from "./toLocalDateTime";
 
-export default (date, timezoneOffset = 0, local = true) => {
+export default (
+  date: string | Date,
+  timezoneOffset = 0,
+  local = true
+): Date => {
   const newDate = new Date(date);
   newDate.setHours(24, 0, 0, 0);
 
diff --git a/date/monthNames.js b/date/monthNames.js
index c870f165..bfb0bc7e 100644
--- a/date/monthNames.js
+++ b/date/monthNames.js
@@ -1,4 +1,4 @@
-export default [
+export default () => [
   "January",
   "February",
   "March",
diff --git a/date/monthNames.json b/date/monthNames.json
index 31794929..fe7b6a5e 100644
--- a/date/monthNames.json
+++ b/date/monthNames.json
@@ -1,6 +1,6 @@
 {
   "name": "monthNames",
-  "description": "TODO: Fill short description here.",
+  "description": "Returns all months names.",
   "signature": "TODO: Fill type signature here.",
   "examples": [
     {
diff --git a/date/monthNames.md b/date/monthNames.md
index 30f0f68a..958ae407 100644
--- a/date/monthNames.md
+++ b/date/monthNames.md
@@ -1 +1,3 @@
 # monthNames
+
+Returns all months names.
diff --git a/date/monthNames.test.ts b/date/monthNames.test.ts
index 0a888d04..1abda834 100644
--- a/date/monthNames.test.ts
+++ b/date/monthNames.test.ts
@@ -3,7 +3,22 @@
 import monthNames from "./monthNames.ts";
 
 describe("monthNames", () => {
-  it.skip("TODO", () => {
-    expect(monthNames()).toBeDefined();
+  it("Returns all month names", () => {
+    const allMonthNames = [
+      "January",
+      "February",
+      "March",
+      "April",
+      "May",
+      "June",
+      "July",
+      "August",
+      "September",
+      "October",
+      "November",
+      "December"
+    ];
+
+    expect(monthNames()).toEqual(allMonthNames);
   });
 });
diff --git a/date/monthNames.ts b/date/monthNames.ts
index c870f165..909169e7 100644
--- a/date/monthNames.ts
+++ b/date/monthNames.ts
@@ -1,4 +1,4 @@
-export default [
+export default (): string[] => [
   "January",
   "February",
   "March",
diff --git a/date/startOfDay.json b/date/startOfDay.json
index 952fa86a..ef50d412 100644
--- a/date/startOfDay.json
+++ b/date/startOfDay.json
@@ -1,6 +1,6 @@
 {
   "name": "startOfDay",
-  "description": "TODO: Fill short description here.",
+  "description": "Calculates start of a day in provided date.",
   "signature": "TODO: Fill type signature here.",
   "examples": [
     {
diff --git a/date/startOfDay.md b/date/startOfDay.md
index e257769b..ca44b63a 100644
--- a/date/startOfDay.md
+++ b/date/startOfDay.md
@@ -1 +1,3 @@
 # startOfDay
+
+Calculates start of a day in provided date.
diff --git a/date/startOfDay.test.ts b/date/startOfDay.test.ts
index ca6f59c1..a0da3058 100644
--- a/date/startOfDay.test.ts
+++ b/date/startOfDay.test.ts
@@ -3,7 +3,22 @@
 import startOfDay from "./startOfDay.ts";
 
 describe("startOfDay", () => {
-  it.skip("TODO", () => {
-    expect(startOfDay()).toBeDefined();
+  const date = new Date("2018-12-31T13:54:33.232Z");
+
+  it("should handle timezone offsets", () => {
+    const localOffset = -300;
+
+    const start = startOfDay(date, localOffset);
+    const expectedUTC = new Date("2018-12-31T05:00:00.000Z");
+    const expectedLocal = new Date("2018-12-31T00:00:00.000-05:00");
+
+    expect(start).toEqual(expectedUTC);
+    expect(start).toEqual(expectedLocal);
+  });
+
+  it("should handle default local date time display", () => {
+    const localOffset = 0;
+
+    expect(startOfDay(date)).toEqual(startOfDay(date, localOffset));
   });
 });
diff --git a/date/startOfDay.ts b/date/startOfDay.ts
index 82289a4b..fb9c411c 100644
--- a/date/startOfDay.ts
+++ b/date/startOfDay.ts
@@ -1,6 +1,10 @@
 import toLocalDateTime from "./toLocalDateTime";
 
-export default (date, timezoneOffset = 0, local = true) => {
+export default (
+  date: string | Date,
+  timezoneOffset = 0,
+  local = true
+): Date => {
   const newDate = new Date(date);
   newDate.setHours(0, 0, 0, 0);