Day of Year & ISO Week Number

Enter a date (YYYY-MM-DD) to see its weekday, the day of the year (1–365 or 1–366 in a leap year), the ISO 8601 week number and ISO year (the week numbering used in most of the world, where week 1 is the week containing the year's first Thursday), how many days remain in the year, and whether the year is a leap year. All date arithmetic uses UTC midnight to avoid timezone drift. Everything runs locally in your browser.