Date Calculator

2026-01-01 to 2026-03-01 is 59 day(s).

How it works

• Parsed both dates as UTC instants.
• Subtracted the start from the end and divided by 86,400,000 ms per day.
• Rounded to the nearest whole day.

days = (end - start) / 86_400_000

Methodology

1. Parse both dates as ISO 8601 calendar dates in UTC and measure whole days, or add a whole-day offset.
2. Use the real length of each month and year (including leap years) rather than a fixed 30-day month.
3. Report whole days and weeks explicitly; calendar units such as months are never guessed from a day count.

Time model

• Time source: an explicit reference supplied by the visitor.
• Timezone: Dates are treated as calendar dates with no timezone; date-only arithmetic is independent of the visitor's timezone. A date-time input with an offset is read as the instant it denotes.
• Locale: ISO-8601 date input and output only; no locale-specific ordering or month names are applied.
• Daylight saving: Day arithmetic counts whole calendar days, so daylight-saving transitions never change the result.
• Reference date: A start date is required and an end date or day count is supplied explicitly; the tool never defaults to the current date.

Usage

1. Enter a start date.
2. Enter an end date to measure a difference, or a number of days to add.
3. Read the result.

Examples

These are examples, not current values: they use fixed inputs, while the interactive tool computes live results in your browser.

Difference in days

{
  "start": "2026-01-01",
  "end": "2026-03-01"
}

Add 30 days

{
  "start": "2026-01-01",
  "addDays": 30
}

Limitations

Difference vs. duration

A difference answers 'how many whole days elapsed?'. A duration in months or years depends on where you start, which is why this tool reports days and weeks explicitly rather than guessing calendar units.