Question 1

How do I calculate the number of days between two dates?

Accepted Answer

To find the number of days between two dates, subtract the earlier date from the later date. In JavaScript this means converting both dates to milliseconds since the Unix epoch, subtracting, and dividing by 86,400,000 (milliseconds per day). This date calculator does it instantly.

Question 2

How many business days are between two dates?

Accepted Answer

Business days exclude weekends (Saturday and Sunday). To count them, iterate through each day in the range and skip days where getDay() returns 0 (Sunday) or 6 (Saturday). This calculator includes a business days count that optionally accounts for common public holidays.

Question 3

How do I add days to a date?

Accepted Answer

To add days to a date, take the starting date's timestamp in milliseconds and add the number of days multiplied by 86,400,000. JavaScript's Date object handles month-end rollovers and leap years automatically. Use this calculator's 'Add/Subtract Days' mode to get the result date instantly.

Question 4

What is the ISO week number?

Accepted Answer

The ISO week number (ISO 8601) counts weeks from 1 to 52 or 53, where week 1 is the week containing the first Thursday of the year. ISO weeks always start on Monday. Week 1 of a year may begin in late December of the previous year, which is why some dates in early January belong to the previous year's last week.

Question 5

How do I find what day of the week a date is?

Accepted Answer

Enter the date in this calculator and the day of the week is shown instantly. Alternatively, in JavaScript use new Date('YYYY-MM-DD').toLocaleDateString('en-US', { weekday: 'long' }). The Zeller's congruence formula can compute it mathematically without a computer.

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