Date Calculator
Add or subtract days, weeks, months, or years from a start date. Months follow the civil calendar, including short-month overflow. Results update as you type.
1. Enter values
2. Result
Add 10 Days from 2026-01-01
2026-01-11
Sunday
10
Days
3. Summary
How is it calculated?
Days and weeks add fixed calendar ticks. Months and years follow this device’s civil calendar, including short-month overflow — the same rules most business software uses.
Common use cases
Deadlines
Add a number of days to a start date to find a due date.
Warranties
See when a coverage window ends after the purchase date.
Invoices
Compute net-30 style dates without counting hours.
Project plans
Shift a milestone by weeks, months, or years.
Travel windows
Count calendar days before a departure, including weekends.
Contracts
Add or subtract a term from a signature date.
Your data stays private
Date arithmetic runs locally in your browser. Nothing is uploaded or stored on a server.
About adding and subtracting calendar dates
Project plans, invoice due dates, and warranty windows are calendar problems, not raw hour counts. Adding 30 days to 31 January is not the same as adding one month: one month lands on a February date (or the last day of February), while 30 days is a fixed span. This tool uses the JavaScript civil calendar in your browser. Days and weeks add 24-hour calendar ticks. Months and years use setMonth / setFullYear, so 31 January plus one month becomes 3 March in a non-leap year — the same overflow most business software uses.
Use add when you know a start date and a duration: “ten business-unaware days after contract signature.” Use subtract when you know a deadline and need the latest start. Weeks are seven calendar days, including weekends; they are not working days. For weekdays only, open the Working Days Calculator instead.
Time of day is ignored. The result is a date, plus the weekday name in English for a quick check against a printed calendar. Nothing is uploaded. If you need a time zone wall clock, use Time Zone Converter — shifting a date by months does not apply DST rules.
How to shift a date
Pick the start date
Use the contract, birth, or invoice date you want to move from.
Choose add or subtract, then the unit
Days and weeks are fixed lengths. Months and years follow the civil calendar and can overflow short months.
Read the ISO date and weekday
Copy the result, or load an example to see a month overflow. Calculate jumps to the answer.
Stay on this device
The new date is computed in this tab. We never send the start date anywhere.
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Convert a wall-clock date and time between IANA time zones, including DST on that calendar date. Search zones such as Asia/Ho_Chi_Minh. Nothing is uploaded.
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Date Calculator FAQ
No. Days and weeks are fixed lengths (a week is seven calendar days, including weekends). Months and years use the civil calendar. 31 January plus one month overflows to early March in a non-leap year, the same way most business software does. Subtract moves backward with the same rules. The result is an ISO date plus a weekday name.
Not on this page. Adding ten days includes Saturdays and Sundays. For Monday–Friday counts, open Working Days Calculator. For a signed day span between two known dates, use Days Between Dates. Time of day and time zones are ignored here; use Time Zone Converter for wall clocks.
No. Add and subtract run in the browser. Nothing is uploaded or stored on a server. Close the tab to clear the start date.