Adding/Subtracting n amount of minutes, hours or days etc. to/from an existing date object is a common problem in programming. In this article we will learn how to add/subtract n amount of time period to a date instance.
Background📖
JavaScript Date API provides several methods, out of which we are interested in a few:
| Method |
Description |
setFullYear() |
Sets specified year for a date instance according to local time. Note: Don’t get confused with setYear() method. |
setMonth() |
Sets specified month for a date instance |
setHours() |
Sets specified hours for a date instance |
setMinutes() |
Sets specified minutes for a date instance |
setSeconds() |
Sets specified seconds for a date instance |
setMilliseconds() |
Sets specified milliseconds for a date instance |
We can use above methods to add/subtract a given time period to/from a date instance.
Code💻
Let’s us see how to add and subtract given time period in minutes to existing date instance.
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/**
* Adds specified time period in minutes to the given `date` instance.
*
* @param {Date} date Date instance to add minutes
* @param {number} amount Number of minutes to add
*/
function addMinutes(date, amount) {
// Don't change the original date object
const newDate = new Date(date);
return newDate.setMinutes(newDate.getMinutes() + amount);
}
/**
* Subtracts specified time period in minutes from given `date` instance.
*
* @param {Date} date Date instance to subtract minutes
* @param {number} amount Number of minutes to subtract
*/
function subtractMinutes(date, amount) {
// Don't change the original date object
const newDate = new Date(date);
return newDate.setMinutes(newDate.getMinutes() - amount);
}
// Test functionality
let start = new Date();
let startPlus90Min = new Date(addMinutes(start, 90));
let startMinus90Min = new Date(subtractMinutes(start, 90));
// Log test results
console.log('Start Date = ', start);
console.log('Start + 90 min = ', startPlus90Min);
console.log('Start - 90 min = ', startMinus90Min);
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Similar way we can add time periods in other units too.
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/**
* Adds specified time period in given `unit` to the specified `date` instance.
*
* @param {Date} date Date instance to add
* @param {'milliseconds'|'seconds'|'minutes'|'hours'|'days'|'months'|'years'} unit Time period unit
* @param {number} amount Amount to add
*/
function addTimePeriod(date, unit, amount) {
// Check for errors
if (!(date instanceof Date) || typeof amount !== 'number') return date;
// Create new instance of date, don't modify existing date
const newDate = new Date(date);
switch (unit) {
case 'milliseconds': newDate.setMilliseconds(newDate.getMilliseconds() + amount); break;
case 'seconds': newDate.setSeconds(newDate.getSeconds() + amount); break;
case 'minutes': newDate.setMinutes(newDate.getMinutes() + amount); break;
case 'hours': newDate.setHours(newDate.getHours() + amount); break;
case 'days': newDate.setDate(newDate.getDate() + amount); break;
case 'months': newDate.setMonth(newDate.getMonth() + amount); break;
case 'years': newDate.setFullYear(newDate.getFullYear() + amount); break;
}
return newDate;
}
// Test functionality
let start = new Date();
console.log('Start Date = ', start);
console.log('Start Date + 50ms = ', addTimePeriod(start, 'milliseconds', 50));
console.log('Start Date + 50s = ', addTimePeriod(start, 'seconds', 50));
console.log('Start Date + 50m = ', addTimePeriod(start, 'minutes', 50));
console.log('Start Date + 5hr = ', addTimePeriod(start, 'hours', 5));
console.log('Start Date + 5 day = ', addTimePeriod(start, 'days', 5));
console.log('Start Date + 5 month = ', addTimePeriod(start, 'months', 5));
console.log('Start Date + 5 year = ', addTimePeriod(start, 'years', 5));
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Note:
I have left the subtraction of dates as an exercise for you. I guess it shouldn’t be difficult now to try.
References
Happy Coding 👨💻
