# Date - 30 seconds of code
Curated collection of useful Javascript snippets that you can understand in 30 seconds or less.
Author: Angelos Chalaris (chalarangelo@gmail.com)
Keywords: javascript, snippets, code, programming

## Date

#### dayOfYear

Gets the day of the year from a `Date` object.

Use `new Date()` and `Date.prototype.getFullYear()` to get the first day of the year as a `Date` object, subtract it from the provided `date` and divide with the milliseconds in each day to get the result. Use `Math.floor()` to appropriately round the resulting day count to an integer.

    const dayOfYear = date =>
      Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);

examples

    dayOfYear(new Date()); // 272

#### formatDuration

Returns the human readable format of the given number of milliseconds.

Divide `ms` with the appropriate values to obtain the appropriate values for `day`, `hour`, `minute`, `second` and `millisecond`. Use `Object.entries()` with `Array.prototype.filter()` to keep only non-zero values. Use `Array.prototype.map()` to create the string for each value, pluralizing appropriately. Use `String.prototype.join(', ')` to combine the values into a string.

    const formatDuration = ms => {
      if (ms < 0) ms = -ms;
      const time = {
        day: Math.floor(ms / 86400000),
        hour: Math.floor(ms / 3600000) % 24,
        minute: Math.floor(ms / 60000) % 60,
        second: Math.floor(ms / 1000) % 60,
        millisecond: Math.floor(ms) % 1000
      };
      return Object.entries(time)
        .filter(val => val[1] !== 0)
        .map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
        .join(', ');
    };

examples

    formatDuration(1001); // '1 second, 1 millisecond'
    formatDuration(34325055574); // '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'

#### getColonTimeFromDate

Returns a string of the form `HH:MM:SS` from a `Date` object.

Use `Date.prototype.toString()` and `String.prototype.slice()` to get the `HH:MM:SS` part of a given `Date` object.

    const getColonTimeFromDate = date => date.toTimeString().slice(0, 8);

examples

    getColonTimeFromDate(new Date()); // "08:38:00"

#### getDaysDiffBetweenDates

Returns the difference (in days) between two dates.

Calculate the difference (in days) between two `Date` objects.

    const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
      (dateFinal - dateInitial) / (1000 * 3600 * 24);

examples

    getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9

#### getMeridiemSuffixOfInteger

Converts an integer to a suffixed string, adding `am` or `pm` based on its value.

Use the modulo operator (`%`) and conditional checks to transform an integer to a stringified 12-hour format with meridiem suffix.

    const getMeridiemSuffixOfInteger = num =>
      num === 0 || num === 24
        ? 12 + 'am'
        : num === 12
          ? 12 + 'pm'
          : num < 12
            ? (num % 12) + 'am'
            : (num % 12) + 'pm';

examples

    getMeridiemSuffixOfInteger(0); // "12am"
    getMeridiemSuffixOfInteger(11); // "11am"
    getMeridiemSuffixOfInteger(13); // "1pm"
    getMeridiemSuffixOfInteger(25); // "1pm"

#### isAfterDate

Check if a date is after another date.

Use the greater than operator (`>`) to check if the first date comes after the second one.

    const isAfterDate = (dateA, dateB) => dateA > dateB;

examples

    isAfterDate(new Date(2010, 10, 21), new Date(2010, 10, 20)); // true

#### isBeforeDate

Check if a date is before another date.

Use the less than operator (`<`) to check if the first date comes before the second one.

    const isBeforeDate = (dateA, dateB) => dateA < dateB;

examples

    isBeforeDate(new Date(2010, 10, 20), new Date(2010, 10, 21)); // true

#### isSameDate

Check if a date is the same as another date.

Use `Date.prototype.toISOString()` and strict equality checking (`===`) to check if the first date is the same as the second one.

    const isSameDate = (dateA, dateB) => dateA.toISOString() === dateB.toISOString();

examples

    isSameDate(new Date(2010, 10, 20), new Date(2010, 10, 20)); // true

#### maxDate

Returns the maximum of the given dates.

Use `Math.max.apply()` to find the maximum date value, `new Date()` to convert it to a `Date` object.

    const maxDate = (...dates) => new Date(Math.max.apply(null, ...dates));

examples

    const array = [
      new Date(2017, 4, 13),
      new Date(2018, 2, 12),
      new Date(2016, 0, 10),
      new Date(2016, 0, 9)
    ];
    maxDate(array); // 2018-03-11T22:00:00.000Z

#### minDate

Returns the minimum of the given dates.

Use `Math.min.apply()` to find the minimum date value, `new Date()` to convert it to a `Date` object.

    const minDate = (...dates) => new Date(Math.min.apply(null, ...dates));

examples

    const array = [
      new Date(2017, 4, 13),
      new Date(2018, 2, 12),
      new Date(2016, 0, 10),
      new Date(2016, 0, 9)
    ];
    minDate(array); // 2016-01-08T22:00:00.000Z

#### tomorrow

Results in a string representation of tomorrow's date.

Use `new Date()` to get today's date, adding one day using `Date.getDate()` and `Date.setDate()`, and converting the Date object to a string.

    const tomorrow = (long = false) => {
      let t = new Date();
      t.setDate(t.getDate() + 1);
      const ret = `${t.getFullYear()}-${String(t.getMonth() + 1).padStart(2, '0')}-${String(
        t.getDate()
      ).padStart(2, '0')}`;
      return !long ? ret : `${ret}T00:00:00`;
    };

examples

    tomorrow(); // 2017-12-27 (if current date is 2017-12-26)
    tomorrow(true); // 2017-12-27T00:00:00 (if current date is 2017-12-26)