# Free online time adder and subtractor

> Need to know what time it will be in 36 hours, or what date falls 90 days before a deadline? Enter a start date and time, choose to add or subtract, enter the duration, and get.

Add or subtract days, hours, minutes, and seconds from any date and time. Get the exact result date and time instantly. Uses UTC internally so the result is always correct regardless of daylight saving time.

**Interactive version:** https://onlinecalculator.me/date-time/time-adder/
**Category:** Date & Time

## How to use

1. Enter the start date using the date picker.
2. Enter the start time (leave as 12:00 if only working with dates).
3. Click **Add** or **Subtract** to choose the direction.
4. Enter any combination of days, hours, minutes, and seconds.
5. Click **Calculate** to see the result.
6. Click "Share with my numbers" to copy a link with all your inputs.

## Formula

The calculator converts all inputs to milliseconds:

```
duration_ms = (days × 86400 + hours × 3600 + minutes × 60 + seconds) × 1000
result_ms   = start_ms ± duration_ms
```

The result timestamp is then formatted back to a date (YYYY-MM-DD) and time (HH:MM) in UTC.

## Worked example

**Start:** 2025-06-15 at 22:30 UTC  
**Add:** 1 day, 3 hours, 45 minutes

Duration in ms: (86400 + 10800 + 2700) × 1000 = 99,900,000 ms

Result: **2025-06-17 at 02:15 UTC**

## Notes

- All results are in UTC. If you need local time, add or subtract your timezone offset.
- Adding 1 day always means exactly 86,400 seconds — no DST adjustments.
- For business day calculations that skip weekends, use the Business Days calculator.

## Frequently asked questions

### How does the time adder work?

It converts your start date and time to a UTC timestamp in milliseconds, adds or subtracts the duration (also converted to milliseconds), then formats the result back to a date and time.

### Why does it use UTC?

UTC has no daylight saving time adjustments. Using UTC means adding 24 hours always moves exactly one calendar day, with no surprises near DST boundaries.

### Can I add negative values?

Use the Subtract button instead of entering negative numbers. The calculator handles the direction switch automatically.

### What is the maximum duration I can add?

There is no hard limit — you can add thousands of days if needed.

### How do I find a date 90 days from today?

Enter today's date, set operation to Add, enter 90 in the Days field, and click Calculate.

## Sources

- [24-hour clock](https://en.wikipedia.org/wiki/24-hour_clock) — Wikipedia
- [Coordinated Universal Time (UTC)](https://en.wikipedia.org/wiki/Coordinated_Universal_Time) — Wikipedia

## Related calculators

- [Add/subtract days calculator](https://onlinecalculator.me/date-time/add-subtract-days/) — Add or subtract a specific number of days from any date.
- [Date difference calculator](https://onlinecalculator.me/date-time/date-difference/) — Find the number of days, weeks, or months between two dates.
- [Business days calculator](https://onlinecalculator.me/date-time/business-days/) — Count working days between dates, skipping weekends and holidays.
- [Time duration calculator](https://onlinecalculator.me/date-time/time-duration/) — Calculate the duration between two times in hours, minutes, and seconds.
- [Countdown calculator](https://onlinecalculator.me/date-time/countdown/) — Count down the days, hours, and minutes to any future date.

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Part of [onlinecalculator.me](https://onlinecalculator.me/) — free calculators that run entirely in your browser.
Machine-readable index: https://onlinecalculator.me/llms.txt