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Add or Subtract Days Calculator

Adds a signed number of days to an ISO date and returns the resulting date and its weekday, using Gregorian calendar arithmetic in UTC.

When to use

You need the date that is N days after (or before) a given date, e.g. a deadline 90 days from today or a return date 14 days after departure.

Do not use when: You need the number of days between two known dates (use date-difference) or want to skip weekends when counting (not supported here).

Formula

result_date = start_date + days (calendar days, no weekend skipping); weekday = day of week of result_date

Inputs

Parameter	Type	Unit	Required	Description
`start_date`	date		default today	Date to start from (YYYY-MM-DD); defaults to the current UTC date.
`days`	integer	days	yes	Number of days to add; negative values subtract. Range: ≥ -1000000, ≤ 1000000

Outputs

Output	Type	Description
`result_date`	date	start_date + days.
`weekday`	string	Day of the week of the result date.
`offset_text`	string	The offset written as days and as weeks and days, before or after start_date.

Example

2026-09-23 + 90 days: {"start_date":"2026-09-23","days":90} → {"result_date":"2026-12-22","weekday":"Tuesday","offset_text":"90 days (12 weeks 6 days) after 2026-09-23"}

2026-03-01 − 30 days: {"start_date":"2026-03-01","days":-30} → {"result_date":"2026-01-30","weekday":"Friday","offset_text":"30 days (4 weeks 2 days) before 2026-03-01"}

GET https://tttkmbb.com/api/v1/calculate/add-days?start_date=2026-09-23&days=90

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/add-days (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/add-days · Markdown: https://tttkmbb.com/everyday/add-days.md · JSON definition: https://tttkmbb.com/everyday/add-days.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="add-days"
OpenAPI operationId: calculate_date_plus_days
Freshness: daily. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Gregorian calendar – leap-year rule and month lengths (Wikipedia) (reference)
ISO 8601 – Date and time format (Wikipedia) (reference)

FAQ

Are weekends or holidays skipped?

No. Every calendar day counts. For business-day counting between two dates see date-difference.

Is the start date day 0 or day 1?

Day 0: adding 1 day to Monday gives Tuesday. For an inclusive convention (start date counts as day 1) add one day fewer.

Related calculators

Date Difference Calculator — Inverse operation: days between two dates.
Days Until Calculator — Countdown from today to a target date.