Date Calculator
Calculate days between dates, add or subtract days from a date, and look up day of week, week number, and more. Supports business days.
Calculator Mode
Select Two Dates
Results
Total Days
92
Weeks + Days
13 weeks, 1 days
Business Days
65
Total Hours
2,208
How to Use Date Calculator
- 1
Choose a mode
Select 'Days Between Dates', 'Add/Subtract Days', or 'What Day Is It?' for different calculations.
- 2
Enter dates
Enter one or two dates depending on the mode you selected.
- 3
Adjust options
Toggle business days to exclude weekends, or switch between adding and subtracting days.
- 4
View results
See total days, weeks, business days, day of week, week number, and more.
Frequently Asked Questions
Related Tools
Date Math Is Harder Than You Think
Adding "one month" to January 31 raises an impossible question: is the answer February 28, February 31 (invalid), or March 3? Different software handles this differently — Excel returns February 28, some date libraries return March 2 or 3. Our calculator follows the convention of clamping to the last valid day of the target month, so January 31 + 1 month = February 28 (or 29 in a leap year). Awareness of this ambiguity is critical when calculating contract deadlines and payment due dates.
Business Days and Why They Matter
Business day calculations exclude weekends (Saturday and Sunday) but not public holidays, which vary by country, state, and sometimes city. A "5 business day" shipping estimate from a Friday means the following Friday, not Wednesday. Legal deadlines are particularly sensitive — many courts count business days differently, and some jurisdictions exclude the starting day while others include it. When precision matters, always clarify the counting convention.
ISO Week Numbers Explained
The ISO 8601 week numbering system starts weeks on Monday and defines Week 1 as the week containing the first Thursday of January. This means January 1 can fall in Week 52 or 53 of the previous year — a fact that has caused bugs in countless software systems. The year 2026 has 53 ISO weeks, while most years have 52. Financial reporting, manufacturing schedules, and logistics planning often reference ISO weeks, making this seemingly obscure standard practically important.