Date Format Converter
Enter one date and get it back in every format that matters: ISO 8601, the UK day-first form, the US month-first form, the long written form, plus the day of the year, the ISO week number, the quarter and the Unix timestamp.
Convert a date
Why 03/04/2026 is a genuinely ambiguous date
In the UK it is the third of April. In the US it is the fourth of March. There is no way to tell from the string which was meant, and this is not a trivia point — it causes real errors in contracts, bookings, invoices and spreadsheets every day.
ISO 8601 exists to end the argument. Written 2026-04-03, largest unit first, it has exactly one reading anywhere in the world. It also sorts correctly as plain text, which is why it is the right format for filenames and databases as well as for anything crossing a border.
How to use this calculator
- Pick a date.
- Press Calculate to see it in every common format, with the week and day numbers.
ISO week numbers are not what most people expect
An ISO week starts on Monday, and week 1 is the week containing the first Thursday of the year. That means the first days of January often belong to the previous year's week 52 or 53, and the last days of December can belong to week 1 of the next year.
1 January 2023 fell on a Sunday, so it was in week 52 of 2022. This calculator shows the week's own year alongside the number, and flags it when the two disagree — a detail that quietly breaks weekly reporting when it is missed.
Frequently Asked Questions
What is ISO 8601 date format?
The international standard, written year-month-day as 2026-04-03. Because it goes from the largest unit to the smallest it has only one possible reading anywhere in the world, and it sorts correctly as plain text.
How do I convert between UK and US date formats?
Enter the date once and the calculator shows both. UK format is day first, DD/MM/YYYY; US format is month first, MM/DD/YYYY. Any date where the day is 12 or less is ambiguous between them.
What is the day of the year?
How many days have elapsed since 1 January, counting that day as one. 31 December is day 365, or day 366 in a leap year.
Why does my ISO week number look wrong?
ISO weeks start on Monday and week 1 is the one containing the first Thursday of the year, so early January can belong to the previous year's week 52 or 53. The calculator shows which year the week belongs to and warns you when it differs.
What is a Unix timestamp?
The number of seconds since midnight UTC on 1 January 1970, used widely in software. The figure here is for midnight UTC at the start of the date you entered.