What happens when I add a month to the 31st or to 29 February?

When adding or subtracting months or years lands on a day that does not exist in the target month, the date is clamped to that month’s last valid day — the convention used by most calendars and date libraries. So 31 January plus one month becomes 28 February (or 29 February in a leap year), and 29 February plus one year becomes 28 February in a common year. This keeps results predictable instead of silently rolling into the following month.

How do I find the date a number of days or weeks from now?

Tap “Now” to set the start to the current moment, keep the operation on Add, and enter the number of days or weeks. The calculator returns the exact future date and weekday; switch to Subtract to look backward instead. Weeks are simply seven days each, so “6 weeks” and “42 days” give the same result.

Does it handle leap years correctly?

Yes. Every February 29 within the span is respected, and the leap-year rule follows the Gregorian calendar: a year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. That means adding days across a leap year counts the extra day, and landing on 29 February is only kept when the target year is actually a leap year.

What is the difference between adding “1 month” and adding “30 days”?

Adding one month moves to the same day number in the next month, whatever its length, while adding 30 days always advances exactly 30 calendar days. Because months range from 28 to 31 days, the two usually land on different dates — 1 January plus one month is 1 February, but plus 30 days is 31 January. Use months when you mean calendar months and days when you mean a fixed number of days.

Does the calculator handle the time of day and time zones?

Yes — you can set a time as well as a date, and hours, minutes and seconds are applied exactly, rolling over midnight and into the next day or month when needed. All times use your own device’s local time zone, and results around a daylight-saving change reflect your local wall clock. Leave the time at 00:00 if you only care about whole dates.

Add or Subtract Time Calculator

The Add or Subtract Time Calculator takes a starting date and time and shifts it forward or backward by any mix of years, months, weeks, days, hours, minutes and seconds — instantly showing the resulting date, time and day of the week. It is the fastest way to answer questions like “what date is 90 …

ADD / SUBTRACT TIME

Start date & time

Operation

Amount to add

YEARS

MONTHS

WEEKS

DAYS

HOURS

MINUTES

SECONDS

RESULTING DATE & TIME

Enter a start date to begin

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HOW TO USE

1
Set your start date and time, or tap “Now” to use the current moment.
2
Choose whether to Add or Subtract.
3
Enter any combination of years, months, weeks, days, hours, minutes and seconds.
4
Read the resulting date, time and weekday, then use Share or Embed to save or post it.

HOW TIME IS ADDED AND SUBTRACTED

The calculator applies the calendar units first — years and months — and then the exact durations. When adding or subtracting months or years lands on a day that does not exist in the target month (such as the 31st, or 29 February), the date is clamped to that month’s last valid day, which is the standard convention: 31 January plus one month becomes 28 February (or 29 February in a leap year). Weeks and days are then applied as whole calendar days, and hours, minutes and seconds as exact time, so a result like “90 minutes later” or “three days earlier” lands precisely where you expect, rolling over month and year boundaries automatically.

WHAT PEOPLE USE IT FOR

Deadlines & due dates

Add a notice period, warranty, or “net 30” term to a start date to find the exact due date.

Appointments & reminders

Work out the date 6 weeks after a treatment, or 90 days before an event, in one step.

Contracts & renewals

Project a lease, subscription or trial end date by adding months or years to the start.

Everyday planning

Find “what date is 100 days from now?” or “what was the date 10 weeks ago?” instantly.

FREQUENTLY ASKED QUESTIONS

Enter a starting date and time, pick Add or Subtract, then type how much time to apply across any of the seven units. The calculator shifts the date and shows the exact result, including the new day of the week. You can combine units in one go — for example add 1 year, 6 months and 2 weeks together — and it handles all the month-length and leap-year details for you.

Pick a start moment and choose Add or Subtract.
Combine years, months, weeks, days, hours, minutes and seconds in a single step.
The result shows the new date, time and weekday.
Month and year rollovers are handled automatically.

NOTES & REFERENCES

Years and months are applied first; a non-existent target day (e.g. 31st or 29 Feb) is clamped to the month’s last day.
Weeks and days are added as whole calendar days; hours, minutes and seconds as exact time.
Leap years follow the Gregorian rule (divisible by 4, except centuries not divisible by 400).
Times use your device’s local time zone; results across a daylight-saving change reflect local wall-clock time.

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Last updated: — · Calendar-accurate with full leap-year handling · Verified by the EagleCalculator team · Eagle-eyed accuracy for every calculation.