🧮 Date Calculators
Calculate the difference between dates, add or subtract days, count business days, and find weeks between dates. All tools handle leap years, month boundaries, and time zones accurately.
How Date Calculations Work
All date arithmetic in our tools is based on the Gregorian calendar — the internationally accepted civil calendar used by most of the world since the late 16th century. Dates are internally represented as milliseconds since the Unix epoch (January 1, 1970, 00:00:00 UTC), which makes arithmetic straightforward: subtract two timestamps to get a duration, add milliseconds to get a future date.
The key complexity in date calculation comes from irregular month lengths (28, 29, 30, or 31 days), leap years, and time zone offsets. Our calculators handle all of these correctly using JavaScript's built-in Date API, which is specification-compliant and widely tested.
Leap Year Rules
The Gregorian calendar leap year rule is: a year is a leap year if it is divisible by 4, EXCEPT for century years (ending in 00), which must be divisible by 400. So:
- 2024 — divisible by 4 → leap year (366 days)
- 2100 — divisible by 4, but a century year not divisible by 400 → NOT a leap year
- 2000 — century year divisible by 400 → leap year
- 1900 — century year not divisible by 400 → NOT a leap year
This rule keeps the calendar synchronized with Earth's 365.2425-day solar year. The actual year length is 365 days, 5 hours, 48 minutes, and 45 seconds — the leap year system compensates for the extra ~6 hours per year.
Business Days vs. Calendar Days
Calendar days count every day including weekends. Business days (working days) count only Monday through Friday. For project planning, contract deadlines, and financial calculations, business days are often the relevant unit. A 30-calendar-day project contains approximately 21–22 business days depending on where weekends fall.
Many legal and financial deadlines are expressed in business days: "payment due within 10 business days," "notice required 5 business days before termination." Converting between calendar and business days requires knowing the specific dates involved, not just the count, because weekend distribution varies. Our business days calculator handles this precisely.
Practical Uses for Date Difference Calculations
Date calculations arise in many everyday contexts: calculating age (days, months, and years since birth), figuring out how many days until a deadline or event, determining whether a contract is still within its warranty period, calculating rental durations for billing, figuring out due dates for projects or pregnancies (typically 280 days or 40 weeks from last menstrual period), and planning retirement or savings goals based on years to target date.
Frequently Asked Questions
How do I calculate the number of days between two dates?
Subtract the earlier date from the later date in milliseconds and convert: (Date2 - Date1) / (1000 × 60 × 60 × 24). For whole days, floor the result. Our calculator also shows weeks, months, and years.
How are business days calculated?
Business days exclude Saturday and Sunday. Count only Monday–Friday in the date range. A standard work year has approximately 260 business days (52 weeks × 5 days). Public holidays can further reduce this count.
How does adding days to a date work across month boundaries?
JavaScript's Date object automatically rolls over month and year boundaries. January 31 + 1 day = February 1. February 28 + 1 day = March 1 (non-leap year) or February 29 (leap year). Month lengths and leap years are handled automatically.
What is a leap year and how does it affect date calculations?
Leap years add February 29, occurring every 4 years — except century years, which must be divisible by 400. 2000 and 2400 are leap years; 1900 and 2100 are not. A year from a date may be 365 or 366 days depending on whether Feb 29 falls in the range.
How do I calculate the number of weeks between two dates?
Divide total days by 7 and floor the result for complete weeks. 100 days = 14 complete weeks + 2 days. Use Math.ceil for partial weeks. Our weeks calculator shows both complete weeks and remaining days.