Date Calculator

What this calculator does

The Date Calculator works in two modes. Date difference reports how much time separates two calendar dates, both as a calendar breakdown of years, months, and days and as a total number of days and weeks. Add or subtract days moves a starting date forward or backward by a chosen number of years, months, weeks, and days.

Both modes use the Gregorian calendar and interpret every date in UTC. That keeps the result stable regardless of the device time zone and avoids the local-time drift that can move a date by one day near midnight.

How to use this calculator

1. Choose the Date difference or Add / subtract days mode.
2. For a difference, enter a start date and an end date. Choose whether the end date should count as a full day.
3. For an offset, enter a start date, choose Add or Subtract, and enter the years, months, weeks, and days.
4. Click Calculate.

The default difference example compares January 1, 2026, with December 31, 2026. Excluding the end day gives 364 total days, which is 0 years, 11 months, and 30 days, or 52 weeks. The default offset example starts on January 15, 2026, and adds one month and fifteen days, producing March 2, 2026.

Date formulas

Difference in days = end date - start date
Calendar difference = complete months + remaining days
Offset result = start date + years + months + weeks + days

Months are applied before weeks and days. When the target month is shorter than the starting day, the result is clamped to the last valid day. For example, January 31 plus one month becomes February 28 in a common year and February 29 in a leap year.

Including the end day adds one day to the counted interval. That option is useful when both the first and last day of an event should be counted, such as a multi-day conference or a hotel stay.

What the results mean

The calendar breakdown is the human-readable version of the interval. It counts whole months from the start date and then reports the leftover days. Total days is the exact elapsed count, and total weeks converts that count into full seven-day weeks plus any remaining days.

For an offset, the result panel shows the resulting date, its weekday, and a summary of the offset that was applied. Submitting an offset of zero returns the original date, which is a useful sanity check.

Assumptions and limitations

• Dates must use YYYY-MM-DD and must exist in the Gregorian calendar.
• The end date must be on or after the start date in difference mode.
• Offsets must be non-negative whole numbers; use the Subtract direction to move backward.
• Leap years are handled automatically. Leap seconds and time-of-day changes are outside the scope of this page.
• Business-day counts and holiday exclusions are not included here.

Frequently asked questions

Why does the calendar difference show 11 months and 30 days for almost a full year?

The calendar breakdown counts complete month anniversaries from the start date. January 1 to December 31 does not quite reach the January 1 anniversary, so it is reported as 11 months and 30 days, even though the total is 364 days.

How should I count the end date?

Enable "Yes, count the end date" when both the first and last day matter. For example, a two-day event on June 1 and June 2 has a one-day gap but a two-day inclusive duration.

What happens when I add one month to January 31?

The result is clamped to the last day of the shorter month. In a common year it is February 28; in a leap year it is February 29. This matches the usual calendar convention for month arithmetic.

Reference tables

Fixed-date US federal holidays

2026 US federal holidays

When a fixed-date holiday falls on a weekend, the federal government observes it on the nearest weekday. State, local, and school holidays may differ.

Source

Reference: Calculator.net — Date Calculator.