Online Calculators FreeReference Manual

Date & Time Calculator

One page for everyday date and time math: add or subtract days, weeks, months or years, days until or since a date, day of week, day of year, ISO 8601 week number, leap years, Monday–Friday work days, clock-time spans, time-card hours, and Unix timestamps.

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Saved & recent
CalculatorFORM DATE TIME CALCULATOR
Quick set
RESULT
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One page, every date chore

Pick the task from the dropdown and the form adapts: two fields for a countdown, one for a weekday, three for a time card. Every member shows its substituted formula beneath the board so you can check the arithmetic by hand.

How the calendar math works

All dates are parsed at UTC midnight and counted as whole calendar days, so a “day” is always a day — no 23- or 25-hour surprises. Calendar months are real months: adding a month to January 31 lands on February 28 (or 29), never on March 3. The week-number member implements the actual ISO 8601 rule — week 1 is the week containing the year’s first Thursday, which is why Dec 30, 2024 belongs to 2025-W01. Clock-time tools read wall-clock HH:MM and wrap past midnight whenever the end time is earlier than the start.

Worked example

  1. Add 30 days to January 15, 2026 with the “Add days” member.
  2. 2026-01-15 + 30 days = 2026-02-14 — a Saturday.
  3. Switch to “Work days”: Jun 1 (Mon) to Jun 5 (Fri) 2026, end included.
  4. 5 calendar days, 0 weekend days skipped → 5 work days.

How to use it

  1. Choose the date or time task from the dropdown.
  2. Fill the fields that appear — dates are YYYY-MM-DD, clock times HH:MM.
  3. Read the big result, the detail rows, and the substituted formula.
  4. Use Quick set to load a worked preset, then copy, print, or share the link.

Frequently asked questions

How do I add days to a date?

Pick “Add days to a date”, enter the start date and the number of days. January 15, 2026 plus 30 days is February 14, 2026. The tool works across month ends, year ends, and leap-day February 29 without drift because it adds real calendar days at UTC midnight.

Why does January 31 plus one month give February 28?

February has no 31st. Calendar-aware libraries either overflow into March 3 or clamp to the month’s last day; this tool clamps (Jan 31 + 1 month = Feb 28 in 2026, Feb 29 in a leap year) and flags the clamp in the rows so you know it happened.

What is an ISO week number?

ISO 8601 defines week 1 as the week containing the year’s first Thursday, with weeks running Monday to Sunday. Consequences: Jan 1 is often week 52 or 53 of the previous year, and Dec 28–31 can already be week 1 of the next year. This tool implements the real Thursday rule and reports the ISO year, which can differ from the calendar year.

Do the work-day counts include holidays?

No — they count Monday-through-Friday only. If a public holiday falls inside your span, subtract it yourself: the rows show the weekend days skipped so you can see exactly what the count includes. Use the include/exclude toggle when the end date itself matters (e.g., leave requests).

How does the overnight time span work?

Enter clock times as HH:MM. If the end time is earlier than or equal to the start, the tool assumes the shift crossed midnight and adds 24 hours — so 22:00 to 06:00 is 8h 0m. The time-card member uses the same rule before deducting your unpaid break and reporting decimal payroll hours.

What is a Unix timestamp?

The number of seconds elapsed since January 1, 1970 00:00:00 UTC (the “epoch”), ignoring leap seconds. 1,793,491,200 is November 1, 2026 at midnight UTC. This member converts in both directions — and accepts negative timestamps for pre-1970 dates.