Time Addition & Subtraction Calculator

Time addition and subtraction calculator for dates and schedules

This calculator adds or subtracts a duration from a starting date and time. It is built for real planning use: shifting a meeting, adjusting a deadline, estimating when a task finishes, or working out a future or past timestamp. You enter a start date, a start time, and a duration in days, hours, and minutes. Choose whether you want to add or subtract the duration, then calculate.

The main output is the resulting date and time. That is the exact point in time you land on after applying the duration. The result is shown in a readable format and includes the day of the week, which is useful when you are planning workdays, deliveries, travel legs, or recurring routines. Under that, the calculator also shows the duration expressed as totals in minutes and seconds so you can copy it into other tools or sanity-check long durations quickly.

Use this when you already know the start moment and you want the end moment. If you are planning backwards, switch the operation to subtract. This helps when you have a fixed deadline and want to know the latest safe start time. It is also useful for audit trails and record keeping where you need to compute a timestamp relative to another timestamp.

Assumptions and how to use this calculator

• Enter dates as YYYY-MM-DD and times as HH:MM in 24-hour format (for example 2025-12-13 and 14:30).
• The calculation uses your device’s local time zone and local calendar rules.
• Days are treated as calendar days (24-hour blocks). For business-day counting you need a different tool.
• Hours and minutes can overflow normally (for example 90 minutes becomes 1 hour 30 minutes internally).
• If all duration fields are blank or zero, the result is simply the start date and time.

Common questions

What can I use this calculator for?

Any time you need to shift a timestamp by a known amount. Common cases include project planning, reminders, travel planning, study schedules, and operational cutoffs like “two days and six hours before dispatch.” You can also use it to reverse-plan by subtracting a duration from a known deadline.

Does it handle crossing months and years correctly?

Yes. If your duration pushes the result into a different month or year, the date rolls over naturally. This is exactly why a calculator like this is useful, because doing it mentally is error-prone around month ends, year ends, and leap years.

What happens with daylight saving time changes?

The calculator uses your device’s local time rules. If your local time zone observes daylight saving time, adding a duration across a DST boundary can shift the clock by an extra hour in either direction depending on the change. If you need absolute UTC calculations, use a UTC-based tool and work in ISO timestamps.

Can I use decimals in the duration fields?

No. Enter whole numbers in days, hours, and minutes. If you have fractional hours, convert them to minutes (for example 1.5 hours is 90 minutes). This keeps inputs predictable and avoids rounding surprises.

Why does it show totals in minutes and seconds?

Total minutes and seconds are useful when you need a single-number duration for another system, or when you want a quick consistency check. It also helps compare durations (for example two alternative schedules) without manually converting days and hours into one unit.