Working with dates in JavaScript without dependencies

dates.js

//String + Date Format

const datePattern = /^(\d{2})-(\d{2})-(\d{4})$/
const [, month, day, year] = datePattern.exec('12-25-1995')
new Date(`${month}, ${day} ${year}`)
// => "1995-12-24T13:00:00.000Z"


//String + Time Format

const datePattern = /^(\d{4})-(\d{2})-(\d{2})\s(\d{1,2}):(\d{2})$/
const [, year, month, day, rawHour, min] = datePattern.exec('2010-10-20 4:30')
new Date(`${year}-${month}-${day}T${('0' + rawHour).slice(-2)}:${min}:00`)
// => "2010-10-19T17:30:00.000Z"


//Millisecond / Second / Minute / Hour

new Date().getSeconds()
// => 49
new Date().getHours()
// => 19
new Date(new Date().setSeconds(30))
// => "2018-09-09T09:12:30.695Z"
new Date(new Date().setHours(13))
// => "2018-09-09T03:12:49.695Z"


//Date of Month

new Date().getDate()
// => 9
new Date().setDate(4)
// => "2018-09-04T09:12:49.695Z"


//Day of Week

new Date().getDay()
// => 0 (Sunday)
new Date().setDate(new Date().getDate() - 14)
// => "2018-08-26T09:12:49.695Z"


//Day of Year

Math.floor((new Date() - new Date(new Date().getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24)
// => 252


//Week of Year

const day = new Date()
const MILLISECONDS_IN_WEEK = 604800000
const firstDayOfWeek = 1 // monday as the first day (0 = sunday)
const startOfYear = new Date(day.getFullYear(), 0, 1)
startOfYear.setDate(startOfYear.getDate() + (firstDayOfWeek - (startOfYear.getDay() % 7)))
const dayWeek = Math.round((day - startOfYear) / MILLISECONDS_IN_WEEK) + 1
// => 37

const day = new Date()
const week = 24
const MILLISECONDS_IN_WEEK = 604800000
const firstDayOfWeek = 1 // monday as the first day (0 = sunday)
const startOfYear = new Date(day.getFullYear(), 0, 1)
startOfYear.setDate(startOfYear.getDate() + (firstDayOfWeek - (startOfYear.getDay() % 7)))
const dayWeek = Math.round((day - startOfYear) / MILLISECONDS_IN_WEEK) + 1
day.setDate(day.getDate() - (dayWeek - week) * 7)
day.toISOString()
// => "2018-06-10T09:12:49.794Z


//Days in Month

new Date(2012, 02, 0).getDate()
// => 29


//Maximum / Minimum of the given dates

const array = [
  new Date(2017, 4, 13),
  new Date(2018, 2, 12),
  new Date(2016, 0, 10),
  new Date(2016, 0, 9),
]

new Date(Math.max.apply(null, array)).toISOString()
// => "2018-03-11T13:00:00.000Z"
new Date(Math.min.apply(null, array)).toISOString()
// => "2016-01-08T13:00:00.000Z"


//Add the specified number of days to the given date.

const now = new Date()
now.setDate(now.getDate() + 7)
// => "Sun Sep 16 2018 09:12:49"


//Subtract the specified number of days from the given date.

new Date(new Date().getTime() - 1000 * 60 * 60 * 24 * 7)
// => Sun Sep 09 2018 09:12:49


//Return time from now.

new Intl.RelativeTimeFormat().format(-4, 'day')
// => "4 days ago"


//Get the unit of time between the given dates.

new Date(2007, 0, 27) - new Date(2007, 0, 29)
// => -172800000
Math.ceil((new Date(2007, 0, 27) - new Date(2007, 0, 29)) / 1000 / 60 / 60 / 24)
// => -2


//Check if a date is before another date.

new Date(2010, 10, 20) < new Date(2010, 10, 21)
// => true


//Check if a date is the same as another date.

new Date(2010, 9, 20).valueOf() === new Date(2010, 9, 21).valueOf()
// => false
new Date(2010, 9, 20).valueOf() === new Date(2010, 9, 20).valueOf()
// => true
new Date(2010, 9, 20).getTime() === new Date(2010, 9, 20).getTime()
// => true
new Date(2010, 9, 20).valueOf() === new Date(2010, 9, 20).getTime()
// => true
new Date(2010, 9, 20).toDateString().substring(4, 7) === new Date(2010, 9, 21).toDateString().substring(4, 7)
// => true


//Is Leap Year
new Date(2000, 1, 29).getDate() === 29
// => true


//Check if a variable is a native js Date object.

new Date() instanceof Date
// => true