# Finding day of week from date (day, month, year) in JavaScript

We are required to write a JavaScript function that takes in three argument, namely:day, month and year. Based on these three inputs, our function should find the day of the week on that date.

For example: If the inputs are −

day = 15,
month = 8,
year = 1993

## Output

Then the output should be −

const output = 'Sunday'

## Example

The code for this will be −

const dayOfTheWeek = (day, month, year) => {
// JS months start at 0
return dayOfTheWeekJS(day, month - 1, year);
}
function dayOfTheWeekJS(day, month, year) {
const DAYS = [
'Sunday',
'Monday',
'Tuesday',
'Wednesday',
'Thursday',
'Friday',
'Saturday',
];
const DAY_1970_01_01 = 4;
let days = day − 1;
while (month − 1 >= 0) {
days += daysInMonthJS(month − 1, year);
month −= 1;
}
while (year − 1 >= 1970) {
days += daysInYear(year − 1);
year −= 1;
}
return DAYS[(days + DAY_1970_01_01) % DAYS.length];
};
function daysInMonthJS(month, year) {
const days = [
31, // January
28 + (isLeapYear(year) ? 1 : 0), // Feb,
31, // March
30, // April
31, // May
30, // June
31, // July
31, // August
30, // September
31, // October
30, // November
31, // December
];
return days[month];
}
function daysInYear(year) {
return 365 + (isLeapYear(year) ? 1 : 0);
}
function isLeapYear(year) {
return year % 4 === 0 && year % 100 !== 0 || year % 400 === 0;
}
console.log(dayOfTheWeek(15, 8, 1993));

## Explanation

We want to count how many days until the given date. To do so, we can start at the infamous Unix time 0 (Thursday 1970−01−01) and go from there −

• Count days in complete years

• Count days in month for the incomplete year

• Remaining days for the incomplete month

## Output

And the output in the console will be −

Sunday

Updated on: 20-Nov-2020

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