Add or subtract days

Working out deadlines

Many deadlines are written as a number of days after an event. A 30 day return window, a 60 day notice and a 90 day trial all start from a date and run forward.

Enter the event date, add the days, and the result is the last day of the period. Check the weekday, because a deadline on a weekend may roll to Monday.

Months and years

Adding months moves the same day number forward. If that day is missing, the tool uses the last day of that month. January 31 plus one month is February 28 or 29.

Adding years works the same way. February 29 plus one year gives February 28 in a common year.

Weeks are always seven days, so eight weeks is always 56 days. Months are not fixed, so six months can be 181, 182, 183 or 184 days.

Counting back

Subtract works the same way in reverse. It answers questions like "what date was 100 days ago" and "when did a 12 month warranty start".

It is also handy for due dates and project plans. Start from the end date and subtract the length of each phase to find when it has to begin.

Why months are not days

Thirty days and one month are not the same thing. A month can be 28, 29, 30 or 31 days, so adding a month lands on a different date than adding thirty days.

Pick the unit that matches the wording of your deadline. A contract that says "within 30 days" needs days, and one that says "within one month" needs months.

When a document does not say, ask which one is meant. The difference can be as much as three days.

Dates that fall on weekends

The result is the plain calendar date, even if it is a Saturday or Sunday. The weekday is shown so that you can see this at once.

Many agreements move a weekend deadline to the next business day. If that applies, take the result and look at the next Monday.

For a deadline counted in work days, use the business days calculator instead. It skips weekends and holidays as it counts.

Planning backwards

Subtracting is the best way to plan a launch. Start from the date you must be ready and take away the length of each phase to find the start of each one.

A wedding, a conference and a house move all work this way. List the steps with their lead times, and subtract them one by one from the big day.

Write each date next to its weekday, and you will spot clashes with weekends and holidays before they matter.

Common date sums

Typical sums are 30, 60, 90 and 180 days. Each one is a standard length for trials, notices and review periods.

Half a year is 26 weeks or six calendar months, which are close but not equal. Check the unit that your document names.

One year after a date is the same day next year, except for February 29, where the tool uses February 28.