Tinker 
How to use the date calculator
Choose a start date, then select Add to move forward in time or Subtract to go back. Fill in any combination of years, months, weeks and days; leave the rest at zero. The result date appears instantly, together with its day of the week and the number of calendar days between the start and the result.
Typical uses: finding a deadline 30 or 90 days from today, working out when a 6-month notice period ends, checking the date 2 weeks before an event, or counting back 45 days from a due date.
How the result is calculated
Years and months are applied first, because they are calendar units of varying length. The calculator moves the month forward or back by (years × 12 + months) and keeps the same day of the month. If that day does not exist in the target month, it uses the last day of that month instead, so 31 January plus 1 month gives 28 February, or 29 February in a leap year.
Weeks and days are then added as exact day counts: weeks × 7 + days. Leap years, months of 28 to 31 days and year boundaries are all handled automatically. Every date is treated as a whole calendar day, so time zones and daylight saving changes do not shift the answer.
Worked example
Start on Monday, 15 January 2024 and add 90 days. There are 16 days left in January (to the 31st), leaving 74. February 2024 has 29 days because 2024 is a leap year, leaving 45. March uses 31 more, leaving 14, so the result is 14 April 2024.
90 days is 12 weeks and 6 days. Twelve full weeks land on a Monday again, and 6 more days make it a Sunday, which is exactly what the calculator shows: 2024-04-14, Sunday, 90 calendar days.
Practical tips
Contracts, notice periods and visas sometimes count the start day as day 1 rather than day 0. This tool counts the start date as day 0, so adding 1 day gives tomorrow. If your rule includes the start day, subtract one day from your total.
This tool counts calendar days, including weekends and public holidays. If a deadline is stated in business or working days, the actual date will be later. Remember too that adding 1 month and adding 30 days are not the same thing: the month method follows the calendar, while the day method is a fixed count.
Frequently asked questions
What is 1 month after 31 January?
February has no 31st, so the calculator uses the last day of February: 28 February in a normal year, or 29 February in a leap year. This is the convention most calendars and spreadsheets use.
Is adding 3 months the same as adding 90 days?
Not quite. Three calendar months can be 89 to 92 days depending on which months are involved. Use the months field when a rule says months, and the days field when it says days.
Does the calculator include the start date?
The start date is day 0. Adding 1 day gives the next day, and adding 7 days gives the same weekday one week later. If your rule counts the start date as day 1, use one day fewer.
Does it skip weekends or holidays?
No. It counts every calendar day. Holidays differ by country and year, so business-day deadlines need to be checked against your local calendar.
What happens when I add a year to 29 February?
The next year has no 29 February, so the result is 28 February. For example, 29 February 2024 plus 1 year gives 28 February 2025, which is 365 days later.