Days are exact, months are not
Adding days moves the date by an exact number of calendar days, whatever the months in between. 30 days after January 15 is February 14. 30 days after February 15, 2024 is March 16 because 2024 has a February 29. That makes a day count the safest way to say how far apart two dates are.
Leap years make a year 365 or 366 days. January 1, 2024 plus 365 days is December 31, 2024, not January 1, 2025, because 2024 has 366 days. Starting from January 1, 2023, 365 days does reach January 1, 2024.
Is tomorrow day 1?
Here the day after the start date is day 1, so 1 day after Monday is Tuesday and 7 days after Monday is the next Monday. This is how spreadsheets add days too: a date plus 7 is a week later. If a rule counts the start day itself as day 1, as some notices and legal periods do, add one day fewer than the rule says.
Calendar days or working days
This page counts every day. 30 calendar days is 4 weeks and 2 days. 30 working days is about 6 weeks, because weekends and holidays are skipped. If the wording says business days or working days, use the working days calculator, where you can set your own days off and holidays.
Common day counts
Windows of 14, 30, 60 and 90 days turn up in refund and return policies, payment terms such as net 30, trial periods, notice periods and permit deadlines. For a quick answer press the matching button. For a deadline that matters, read the wording for which day is counted first, whether weekends and holidays are skipped, and what happens if the last day falls on a day off.