The month-by-month method
To add days by hand, use up the rest of the current month first, then take whole months until you can't, then finish with the days that are left.
Say you start on October 5, 2026 and add 90 days. October has 31 days, so 26 days take you to October 31. That leaves 64. November uses 30, leaving 34. December uses 31, leaving 3. You land on January 3, 2027.
Subtracting works the same way in reverse. From October 5, go back 5 days to September 30, then keep peeling off whole months.
- Days left in the start month = days in that month - start day
- Subtract whole months from the remaining count
- The leftover is the day number in the final month
Why "a month" and "30 days" give different answers
A month is a calendar step, not a number of days. One month after October 5 is November 5, which is 31 days later. One month after February 5 is March 5, only 28 days later in 2026.
So when a document says "within 30 days," count days. When it says "within one month," move to the same date next month. Mixing the two is the most common date mistake, and it can put you a day or two off.
Weeks are the easy case
A week is always seven days, so adding weeks never runs into month lengths. Multiply by seven and add the days. Better still, whole weeks always land on the same weekday you started on.
That makes weeks a nice check. Twelve weeks after Monday, October 5, 2026 must be a Monday, and it is: December 28, 2026.
What to do at the end of a month
Adding months breaks when the target month is too short. January 31 plus one month has no February 31, so you need a rule. The usual one picks the last day that exists, February 28 in 2026.
Some billing and payroll systems use a different idea: if you start on the last day of a month, you stay on the last day. Under that rule, February 28, 2026 plus one month is March 31, not March 28. Neither is wrong. Use whichever rule your contract or system states.
Spreadsheet formulas that do it for you
In Excel or Google Sheets, dates are numbers, so =A1+90 adds 90 days and =A1-45 goes back 45. For today, use =TODAY()+90.
For months, use =EDATE(A1, 6) to move six months forward or =EDATE(A1, -6) to go back. It keeps the same day of the month, so check its answer by hand when you start on the 29th, 30th or 31st. For a quick answer without a spreadsheet, our date calculator handles days, weeks, months and years and shows the weekday too.
Formula notes
Add days: =A1 + 90Days from today: =TODAY() + 90Add months: =EDATE(A1, 6)
Mistakes to watch for
Most wrong dates come from one of a few slips.
- Assuming every month has 30 days.
- Counting the start day as day 1 when the rule says to start the next day.
- Forgetting February 29 in a leap year like 2028.
- Adding months as 30-day blocks.
- Using calendar days when the rule says business days.
Frequently asked questions
Do you count the starting day when adding days?
In plain date math, no. Adding 1 day to Monday gives Tuesday. Some legal and contract rules say this explicitly, so check yours if a deadline depends on it.
How do I count back 30 days from a date?
Subtract the day number to reach the end of the previous month, then keep subtracting that month's length. From October 5, 30 days back is September 5, because September has 30 days.
What is the Excel formula to add months to a date?
Use =EDATE(start_date, months). A negative number of months moves backward.
Does a leap year change date calculations?
Only when your range crosses February in a leap year. Then February has 29 days, so the count lands one day earlier than it would in a common year.
References
These sources support the method or guidance used for Date Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Date Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.