onlinecalculator.me

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.

Operation
About this calculator

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

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.