How to find the difference between two dates

Short answer

Subtracting dates gives the difference in milliseconds. Divide it by the number of milliseconds in a day:

const start = new Date(2025, 0, 1);
const end = new Date(2025, 2, 15);
const msPerDay = 24 * 60 * 60 * 1000;
 
console.log(Math.round((end - start) / msPerDay));

Output:

Adding days to a date

setDate() rolls over into the next month when the day count exceeds the current month:

const date = new Date(2025, 0, 30);
date.setDate(date.getDate() + 5);
 
console.log(date.toLocaleDateString('ru-RU'));

Output:

04.02.2025

Hours and minutes

Divide the difference by the number of milliseconds in an hour or a minute:

const start = new Date(2025, 0, 1, 9, 15);
const end = new Date(2025, 0, 1, 17, 45);
const minutes = (end - start) / 60000;
 
console.log(Math.floor(minutes / 60) + ' h ' + (minutes % 60) + ' min');

Output:

8 h 30 min

Common mistake

Because of the switch to daylight saving time a day may last 23 or 25 hours. Round the number of days with Math.round(), otherwise you may get 72.96 instead of 73. Comparing dates in UTC avoids the problem completely:

const start = Date.UTC(2025, 2, 1);
const end = Date.UTC(2025, 3, 1);
 
console.log((end - start) / 86400000);

Output:

Thanks for reading!

Short answer

Adding days to a date

Hours and minutes

Common mistake

Functions used in this recipe

Thanks for reading!