Free Date & Time Calculators
Free online date and time calculators: exact age, days between dates, date arithmetic, Unix timestamp converter, and ISO week numbers. No sign up, no account — runs in your browser.
Free date and time tools covering the calculations people reach for most often — exact age, days between two dates, adding or subtracting time from a date, Unix timestamp conversion, and ISO week numbers. Each tool runs entirely in your browser with no account required.
Whether you need to know how many days until a deadline, what ISO week number a date falls in, or how to convert a Unix timestamp to a readable date, these calculators give you a precise answer instantly.
About each calculator
Age calculator
The age calculator gives you your exact age in years, months, and days from any date of birth to any reference date — defaulting to today. It handles leap years correctly: a person born on 29 February has a birthday on 28 February in non-leap years for the purposes of age counting. The result also shows total days, weeks, and hours elapsed since birth.
Exact age in full years, months, and days matters more than it might seem. Legal thresholds (driving age, retirement age, insurance eligibility), medical references, and contract terms often specify age precisely. Counting manually across months of different lengths and leap years introduces errors that this calculator eliminates.
Days between dates calculator
The days between dates calculator counts the exact number of calendar days, weeks, months, and business days separating any two dates. Business days exclude weekends (Saturday and Sunday) but do not account for public holidays, which vary by country. The calculator works in both directions — it does not require the start date to be earlier than the end date.
Common uses include: calculating notice periods in employment contracts, counting days until an event or deadline, verifying the length of a subscription or billing cycle, or checking how many working days remain in a project. For anything legally binding, calendar days and business days can produce very different numbers — the calculator shows both.
Date add and subtract calculator
The date calculator adds or subtracts any combination of days, weeks, months, or years from any starting date. Month arithmetic is handled correctly: adding one month to 31 January gives 28 February (or 29 in a leap year), not 3 March. This mirrors the behaviour of calendar systems and the way contract terms, billing cycles, and legal deadlines are computed.
Typical uses: calculating a deadline 30, 60, or 90 days from today; finding the date 6 or 12 months from a contract start; projecting a delivery window; or working out a due date. Because month and year lengths vary, adding 1 month is not the same as adding 30 days — this distinction is important for any date that anchors a legal or financial obligation.
Unix timestamp converter
A Unix timestamp is an integer representing the number of seconds (or milliseconds) that have elapsed since the Unix epoch: 00:00:00 UTC on 1 January 1970. The converter accepts either a seconds-precision or milliseconds-precision timestamp and returns the equivalent date and time in UTC, local time, and ISO 8601 format. It also converts in the reverse direction — any date and time back to its Unix timestamp.
Unix timestamps appear in server logs, API responses, database records, JWT tokens, and virtually every programming language's standard library. JavaScript's Date.now() returns milliseconds; most server-side environments and POSIX functions use seconds. The converter detects which you have entered based on the magnitude of the number — values above 10 billion are treated as milliseconds.
ISO week number calculator
The week number calculator returns the ISO 8601 week number for any date and shows the full year calendar with all 52 or 53 weeks. Under ISO 8601, weeks run Monday to Sunday. Week 1 is defined as the week containing the first Thursday of the year — which means 1 January can fall in week 52 or 53 of the previous year, and 31 December can fall in week 1 of the following year.
ISO week numbers are used extensively in supply chain management, manufacturing, broadcasting, and project planning. Many ERP systems, spreadsheets, and scheduling tools reference weeks by number rather than date range. The calculator also shows the Monday and Sunday dates bounding each week, making it easy to convert between a week number and specific calendar dates.
Which calculator to use
For most date questions, start with the simplest tool that answers it. "How old am I exactly?" → age calculator. "How many days until my deadline?" → days between dates. "What date is 90 days from today?" → date add. "What week number is today?" → week number calculator. "What does this timestamp mean?" → Unix timestamp converter.
The days between dates and date add calculators overlap in purpose but answer different questions. Days between tells you the gap between two known dates. Date add tells you what date you reach after a given interval. If you know both dates, use days between; if you know the start and the interval, use date add.
Frequently asked questions
What is the current ISO week number?
Open the week number calculator and it shows today's ISO 8601 week number instantly, along with the Monday and Sunday dates bounding that week. Week 1 is the week containing the first Thursday of the year, so early January dates can sometimes fall in week 52 or 53 of the previous year.
How do I calculate the number of days between two dates?
Enter a start date and an end date in the days between dates calculator. It returns the exact number of calendar days, weeks, months, and business days separating them. Business days exclude weekends but not public holidays, which vary by country.
What is a Unix timestamp?
A Unix timestamp is the number of seconds (or milliseconds) elapsed since 00:00:00 UTC on 1 January 1970, known as the Unix epoch. It is a universal way to represent a point in time as a single integer, used in server logs, APIs, databases, and most programming languages.
What is the difference between seconds and milliseconds in a Unix timestamp?
Most server-side systems and POSIX functions use seconds. JavaScript's Date.now() returns milliseconds — the same timestamp multiplied by 1,000. A seconds-precision Unix timestamp for mid-2024 is around 1,700,000,000; the milliseconds version is around 1,700,000,000,000. The Unix timestamp converter detects which you have entered automatically.
How is ISO week numbering different from a regular calendar week?
ISO 8601 weeks always run Monday to Sunday, and week 1 is defined as the week containing the year's first Thursday. This means 1 January can be in week 52 or 53 of the prior year, and 31 December can be in week 1 of the following year. Some systems (particularly in the US) define week 1 as simply the week containing 1 January, which produces different numbers around year boundaries.
How do I calculate my exact age in days?
Enter your date of birth in the age calculator and it shows your age in years, months, days, total weeks, and total days. The calculation accounts for leap years, so it is accurate regardless of whether your birthday falls in or near February.
Is adding 1 month the same as adding 30 days?
No. Adding one calendar month always lands on the same day of the following month — 15 March plus one month is 15 April. Adding 30 days to 15 March gives 14 April. The difference grows when months have different lengths. For contract terms, notice periods, and billing cycles, calendar months are almost always what is meant — not a fixed number of days.
Why does 31 December sometimes fall in week 1 of the next year?
Under ISO 8601, if 31 December falls on a Monday, Tuesday, or Wednesday, it is part of the first week of the following year — because that week's Thursday is in January. This is mathematically consistent and ensures every week belongs unambiguously to exactly one year, but it can be surprising if you encounter it for the first time.
How many weeks are in a year?
Most years have 52 ISO weeks. A year has 53 weeks if 1 January falls on Thursday, or if it is a leap year and 1 January falls on Wednesday or Thursday. On average, about 71% of years have 52 weeks and 29% have 53.
Do these calculators work for dates in the past and future?
Yes. All five date and time calculators accept any valid calendar date — past or future. There is no practical limit on range for age, days between, and date add. The Unix timestamp converter covers the full 32-bit and 64-bit signed integer range, so it handles dates from 1901 through 2262.
Do I need an account to use these date calculators?
No. Every calculator on this page runs entirely in your browser. No account, no sign up, no email address required. Nothing is sent to a server — your inputs stay on your device.
From the blog
Epoch Time in APIs and Logs Explained: How to Read and Convert It CorrectlyEpoch time appears everywhere in APIs, databases, and logs, but it is easy to misread. This guide explains what epoch time means, why systems use it, and how to convert it without common mistakes.
How to Calculate Age in Months: A Practical Guide for Babies, Records, and PlanningAge in months matters more often than people think, especially for babies, forms, and milestones. This guide explains how to calculate age in months correctly and avoid common mistakes.
How to Calculate Age Accurately: Why the Answer Changes by ContextCalculating age sounds simple until legal forms, birthdays, leap years, and exact day counts enter the picture. This guide explains how to calculate age correctly and when the details matter.
How Age Works for Leap Year Birthdays: February 29 ExplainedBeing born on February 29 creates age-calculation questions people rarely think about until they matter. This guide explains leap-year birthdays, legal age questions, and how to calculate age correctly.