How to Add Days to a Date in TypeScript (setDate and DST)

Copy the Date, then call setDate(getDate() + days) on the copy: that’s how you add days to a date in TypeScript. JavaScript rolls the month and year over for you, and negative numbers go back in time:

const due = new Date(start);
due.setDate(due.getDate() + 30);

Every result here came from TypeScript 7.0.2 and Node.js v22.22.2 in strict mode, with the time zone set to America/New_York.

In this tutorial, I’ll build a typed helper, show why adding 86,400,000 ms breaks at daylight saving time, and cover UTC and business days.

Add days to a date with setDate()

getDate() returns the day of the month. Past the month’s end, setDate() moves into the next month or year:

function addDays(date: Date, days: number): Date {
  const result = new Date(date);          // copy, so the input stays the same
  result.setDate(result.getDate() + days);
  return result;
}

const show = (d: Date) => d.toDateString();
console.log("Jan 30 + 3: ", show(addDays(new Date(2026, 0, 30), 3)));
console.log("Dec 30 + 5: ", show(addDays(new Date(2026, 11, 30), 5)));
console.log("Feb 28 + 1: ", show(addDays(new Date(2028, 1, 28), 1)));

Output:

Jan 30 + 3:  Mon Feb 02 2026
Dec 30 + 5:  Mon Jan 04 2027
Feb 28 + 1:  Tue Feb 29 2028

The helper copies first because setDate() edits the object in place, a habit I also use when adding minutes to a date. For a single day, see getting tomorrow’s date in TypeScript.

Is adding 86,400,000 ms the same as adding a day?

Many snippets add days * 86400000 to the timestamp. That’s always 24 hours, but you’ll get a 23-hour local day in March and a 25-hour one in November:

const DAY_MS = 86_400_000;
const time = (d: Date) => d.toLocaleString("en-US", { dateStyle: "medium", timeStyle: "short" });

function compare(label: string, start: Date): void {
  const bySetDate = new Date(start);
  bySetDate.setDate(bySetDate.getDate() + 1);
  const byMs = new Date(start.getTime() + DAY_MS);
  const hours = (bySetDate.getTime() - start.getTime()) / 3_600_000;
  console.log(label);
  console.log("  setDate +1:   ", time(bySetDate), `(${hours} hours later)`);
  console.log("  + 86400000 ms:", time(byMs));
}

console.log("Time zone:", Intl.DateTimeFormat().resolvedOptions().timeZone);
compare("Spring forward, Mar 7 2026 11:30 PM", new Date(2026, 2, 7, 23, 30));
compare("Fall back, Nov 1 2026 12:30 AM", new Date(2026, 10, 1, 0, 30));

Output:

Time zone: America/New_York
Spring forward, Mar 7 2026 11:30 PM
  setDate +1:    Mar 8, 2026, 11:30 PM (23 hours later)
  + 86400000 ms: Mar 9, 2026, 12:30 AM
Fall back, Nov 1 2026 12:30 AM
  setDate +1:    Nov 2, 2026, 12:30 AM (25 hours later)
  + 86400000 ms: Nov 1, 2026, 11:30 PM

So use setDate() for calendar days like due dates, and milliseconds only for true durations, as in adding seconds to a date. A UTC server hides this, so test with a TZ value.

Add days to a UTC or date-only string

new Date("2026-03-08") is parsed as midnight UTC, which is still March 7 in New York. Local setDate() then crosses the DST change, so stay in UTC end to end:

const parsed = new Date("2026-03-08");          // date-only string = midnight UTC

const local = new Date(parsed);
local.setDate(local.getDate() + 1);
console.log("local setDate:", local.toISOString());

const utc = new Date(parsed);
utc.setUTCDate(utc.getUTCDate() + 1);
console.log("setUTCDate:   ", utc.toISOString().slice(0, 10));

Output:

local setDate: 2026-03-08T23:00:00.000Z
setUTCDate:    2026-03-09

That slice(0, 10) trick is one of several in formatting dates as YYYY-MM-DD in TypeScript. For user input, run checks for invalid dates first.

Add business days and skip weekends

Shipping dates usually skip Saturdays and Sundays, so step one day at a time and count only weekdays:

function addBusinessDays(date: Date, days: number): Date {
  const result = new Date(date);
  let added = 0;
  while (added < days) {
    result.setDate(result.getDate() + 1);
    const day = result.getDay();               // 0 = Sunday, 6 = Saturday
    if (day !== 0 && day !== 6) added++;
  }
  return result;
}

const ordered = new Date(2026, 10, 20);         // Friday, Nov 20, 2026
console.log("ordered: ", ordered.toDateString());
console.log("+5 business days:", addBusinessDays(ordered, 5).toDateString());

Output:

ordered:  Fri Nov 20 2026
+5 business days: Fri Nov 27 2026

The weekend is skipped, so you get the 27th, not the 25th. To count the gap afterward, use the days between two dates. Holidays need a lookup set.

Working on more date math? Try these next:

• Subtract days from a date in TypeScript
• Calculate yesterday’s date in TypeScript
• Compare dates without time in TypeScript
• Get the month from a date in TypeScript

Frequently asked questions