"What date is 90 days from now?" and "how many days are between these two dates?" are the two most common date questions people run into — and both are harder to get right by hand than they look, thanks to months of different lengths and leap years.
Adding or Subtracting from a Date
Adding days, months, or years to a date sounds simple until you hit a month boundary — adding 45 days to a date near the end of a month means correctly rolling over into the next month (and possibly the next year), accounting for months that have 28, 29, 30, or 31 days depending on which month and whether it's a leap year.
Counting Days Between Two Dates
Going the other direction — given two dates, how many days apart are they — requires the same careful handling of month lengths and leap years, just run in reverse. This is the calculation behind deadline tracking, countdowns, and scheduling.
Calendar Days vs Business Days
Confusing the two is one of the most common scheduling mistakes — always check whether a deadline or estimate is quoted in calendar days or business days before planning around it.
Why Leap Years Complicate Things
February has 29 days roughly one year in four, which shifts the exact day count for any date range spanning a leap-year February compared to a non-leap year — a detail that's easy to get wrong doing the math by hand but trivial for a calculator to handle correctly every time.
Step-by-Step: Do the Date Math
- To add or subtract: enter a start date, choose the operation, and enter days/months/years
- To count between dates: enter a start and end date
- Review the result, including business days and weekends if needed