Date CalculatorAdd or subtract days, weeks, months and years from any date

Find the date any number of days, weeks, months or years before or after a start date, with the weekday, day of the year, ISO week and the exact number of calendar days between the two dates.

  • Month-end rule handled
  • Leap years handled
  • Quick 30/60/90-day picks
  • Chain calculations
Manzoor Ahmad
Designed, developed & tested by Manzoor Ahmad, calculator developer
Last reviewed
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Date calculator

Quick picks

Result date

—

—calendar days apart
—day of the year
—ISO week
—weeks + days
From—
0 days
To—

How do I add days to a date?

Count forward one calendar day at a time, rolling into the next month when you pass the month’s last day. The calculator does this instantly for any mix of years, months, weeks and days. For example, 90 days after November 3, 2026 is Monday, February 1, 2027. Months are added before days. If the target month is shorter, the date falls on its last day: January 31 + 1 month = February 28 (29 in a leap year).

How to use the date calculator

  1. Pick a start date, or tap Today.
  2. Choose Add or Subtract, then enter any combination of years, months, weeks and days. Quick picks fill in common periods such as 30, 60 and 90 days.
  3. Read the result: the date, its weekday, day of the year, ISO week number and the exact number of calendar days between the two dates.
  4. Chain calculations with “Use as new start”, for example to add 6 months and then another 30 days.

The order of operations and the month-end rule

Step 1 Add (or subtract) the years and months togetherStep 2 If that day doesn't exist in the new month, use the month's last dayStep 3 Add (or subtract) the weeks × 7 + days

Examples: Jan 31, 2027 + 1 month = Feb 28, 2027. Mar 31, 2028 − 1 month = Feb 29, 2028 (leap year). Aug 31 + 6 months = Feb 28 or 29. Adding 30 days instead of “1 month” gives a different answer, so choose the unit your contract or prescription uses.

Days to weeks and months, a quick reference

DaysWeeks≈ Months (avg 30.44 days)
7 days1 week0.2 months
14 days2 weeks0.5 months
30 days4 weeks 2 days1.0 months
45 days6 weeks 3 days1.5 months
60 days8 weeks 4 days2.0 months
90 days12 weeks 6 days3.0 months
100 days14 weeks 2 days3.3 months
120 days17 weeks 1 day3.9 months
180 days25 weeks 5 days5.9 months
270 days38 weeks 4 days8.9 months
365 days52 weeks 1 day12.0 months

What people use a date calculator for

Deadlines and notice periods

30-day notices, 60-day return windows and 90-day probation periods. For “business days”, use the business days calculator instead.

Pregnancy and baby dates

40 weeks from the first day of the last period is the standard due-date rule (Naegele’s rule).

Medication supply (pharmacist tip)

A 90-day supply started today runs out 90 calendar days later. Refill a few days early so you don’t run short, especially before holidays.

Travel and visas

“Within 180 days” or “valid for 6 months” are not the same period. Check which unit the rule uses.

Related calculators

Different date questions need different counting rules:

Days Between Dates CalculatorAlready have both dates? Count the days, weeks and months between them.Open Days Between Dates Calculator →Business Days CalculatorNeed working days only? Skip weekends and federal holidays.Open Business Days Calculator →Month CalculatorWorking in whole months? Count or add months with the same month-end rule.Open Month Calculator →Days Until CalculatorTurn the result date into a live countdown.Open Days Until Calculator →

More Date & Countdown calculators in this cluster:

Manzoor Ahmad

Manzoor Ahmad

Calculator Developer · Pharmacist · Founder of DexoCalc

I’m a pharmacist turned web developer with 13+ years of experience building web tools. In pharmacy, “30 days” and “1 month” lead to different refill dates, so I built this calculator to keep them separate, with the month-end rule visible. I tested it against leap years, month ends and dates far in the past.

Published · Last reviewed · Author profile · Report an issue

Date calculator: frequently asked questions

Add 90 calendar days to today’s date. The calculator does it automatically with the ‘+90 days’ quick pick. From November 3, 2026 the answer is Monday, February 1, 2027.

Move forward the given number of months and keep the same day number. If that day doesn’t exist in the new month (for example February 30), the calculator uses the last day of the month.

No. Adding 1 day to Monday gives Tuesday. The start date is day 0, which is the standard way to add time to a date.

Choose ‘Subtract’, then enter the days (or weeks, months, years). For example, 30 days before March 15, 2027 is February 13, 2027.

No. A calendar month is 28 to 31 days. Adding ‘1 month’ keeps the day number, while adding ’30 days’ counts exact days, so the two answers can differ by up to three days.

This calculator counts calendar days. To skip weekends and holidays, use the business days calculator.

The result shows the calendar days between your start and result dates. For any two dates you choose, use the days between dates calculator.

Sources & references

  1. ISO 8601: Date and time format, International Organization for Standardization
  2. Time and Frequency Division, National Institute of Standards and Technology

Calendar arithmetic only. For legal or contractual deadlines, confirm how the governing rule counts days.

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