Date Calculator - Add & Subtract Days
Add or subtract days, weeks, months, and years from any date, calculate the exact duration between two dates in days, weeks, and months, and find the day of the week for any date.
Date Calculator — interactive calculator, computed instantly in your browser.
Date Arithmetic: Adding and Subtracting Days Across Calendars
Adding 90 Days to a Start Date
Key Takeaway: Count raw days on the timeline, then read the calendar date back — never add "one month" as 30 fixed days.
Date arithmetic underpins every deadline: contract terms ("net 30"), project plans ("delivery in 60 days"), subscriptions, and legal windows all specify raw day counts. The trap is mixing fixed-length units (days, weeks) with variable-length ones (months, years) — October has 31 days, February has 28 or 29, and 12 weeks is not the same as 3 months.
1. Adding and Subtracting Days
- Days & weeks: pure timeline arithmetic — 90 days from Oct 5, 2026 = Jan 3, 2027 (crosses year end)
- Months: shift the month field, then clamp the day — Jan 31 + 1 month = Feb 28/29, never Mar 3
- Years: Feb 29 on a non-leap target year clamps to Feb 28
- Weekday shifts by (days mod 7): 90 days = 12 weeks + 6 days → weekday moves forward 6 positions
2. Counting the Difference Between Dates
Convert both dates to a day-number (midnight UTC epoch) and subtract — this is immune to DST and clock anomalies because calendar days, not hours, are being compared. A span of 365 days may be 52 weeks + 1 day, and inclusive counting (both endpoints included) adds exactly one day. The Y/M/D breakdown counts full years, then full months, then leftover days using the end month's length for the borrowed-day adjustment.
3. Practical Deadline Patterns
- Net-30 invoices: issue date + 30 calendar days; business-day variants swap in the business days calculator
- Statutes of limitation: often count "days" as business days with specific exclusion rules — read the statute before using a plain calendar count
- Shipping estimates: "3–5 business days" excludes weekends and holidays; raw calendar days overstate by ~40% on long spans
- Leap-year checkpoints: anything anchored to Feb 29 must define its non-leap behavior (Feb 28 vs Mar 1) — contracts and software both get this wrong
🔗 Deadline & Calendar Planning Tools
Project deadlines, contract terms, and subscription renewals are all written in raw dates — converting them into day counts makes planning concrete. Explore our ⏱️ Date & Time Hub for more chronological utilities.
Working-day deadlines need the business-day view, and travel across zones needs the time zone converter — both linked below.
Frequently Asked Questions (Date Calculator)
Add the days directly to the calendar — 90 days = 12 weeks + 6 days, so the date moves forward 12 full weeks plus 6 extra days. For Oct 5, 2026 that lands on Jan 3, 2027, crossing both a month and year boundary.
Months have different lengths, so a safe addition clamps the day to the target month's last day: Jan 31 + 1 month = Feb 28 (Feb 29 in leap years). Simply adding 30 days would give Mar 2/3 — a different result entirely.
Use total days minus weekend days: a 45-calendar-day span contains roughly 13 weekend days, leaving ~32 weekdays. Our business days calculator applies the exact count including holidays and optional half-days.
No — a calendar year is 365 days but a leap year is 366 (Feb 29), and any span crossing a leap boundary changes by a day. Date arithmetic handles this automatically because it works on calendar dates rather than fixed 365-day assumptions.
Inclusive counting includes both the start and end dates, adding one extra day: Oct 1 to Oct 3 inclusive = 3 days (1, 2, 3), while exclusive difference = 2 days. Contracts specify which convention applies — 'within 3 days of notice' is usually inclusive.