Date Math Is Harder Than It Looks
Adding 30 days to January 1 gives you January 31. Adding one month to January 1 gives you February 1. These are different answers, and both are correct — because "30 days" and "one month" are not the same thing.
Date arithmetic is deceptively tricky because months have different lengths (28, 29, 30, or 31 days), years have different lengths (365 or 366 days), and time zones add another layer of complexity.
Adding and Subtracting Days
Adding days is straightforward — it is pure counting. Add 90 days to March 15, 2026, and you get June 13, 2026. The calculation crosses month boundaries automatically. The only complication is leap years: February 29 exists in leap years but not in regular years.
Common day-counting tasks:
- Deadlines: "The contract expires 90 days from signing"
- Due dates: "Payment is due 30 days after the invoice date"
- Countdowns: "How many days until December 25?"
- Scheduling: "The follow-up appointment is in 14 days"
Adding and Subtracting Months
Month arithmetic has an edge case: what happens when the resulting month does not have enough days? Adding one month to January 31 should give February 31 — but February only has 28 or 29 days.
Most systems handle this by capping to the last day of the resulting month: January 31 + 1 month = February 28 (or 29 in a leap year). This is intuitive but not reversible: February 28 + 1 month = March 28, not March 31.
This asymmetry means that adding a month and then subtracting a month does not always return you to the original date. Be aware of this when calculating recurring dates.
Adding and Subtracting Years
Adding years is simple except around February 29. If you add one year to February 29, 2024 (a leap year), the result is February 28, 2025 (not a leap year, so February 29 does not exist). Like month arithmetic, the day is capped to the last valid day of the resulting month.
Leap Year Rules
A year is a leap year if:
- It is divisible by 4, AND
- It is NOT divisible by 100, UNLESS
- It is divisible by 400
So: 2024 is a leap year. 2100 is not (divisible by 100 but not 400). 2000 was a leap year (divisible by 400). This matters when calculating dates far in the future or past.
Business Days vs Calendar Days
Legal and financial deadlines often specify "business days" — weekdays excluding holidays. Calculating business days requires knowing the holiday calendar for the relevant jurisdiction, which varies by country, state/province, and sometimes industry.
A 10 business day deadline from a Friday is typically the following Friday plus two weeks (skipping two weekends), but holidays may extend it further.
How to Use the Toobits Date Calculator
Enter a starting date and add or subtract days, weeks, months, or years to calculate the resulting date. The calculator handles month-length differences and leap years automatically. Use it for deadline calculation, project planning, or any date arithmetic. Everything runs in your browser.