WhatWeek

Date to Week Number Converter

Convert a date to its week number, week-based year, weekday, week start and end dates, and planning metadata.

Week results use the time zone entered here. Type any IANA time zone, such as Europe/Paris or America/Los_Angeles.
Week system

2027-01-04 falls in ISO Week 1 of 2027 (2027-W01-1), from 2027-01-04 through 2027-01-10.

Date
2027-01-04
Week system
ISO
ISO week date
2027-W01-1
Week-based year
2027
Week number
1
Week starts
2027-01-04
Week ends
2027-01-10
Day of year
4
Quarter
Q1
Weeks remaining
51
Excel formula
=ISOWEEKNUM(DATE(2027,01,04))

Public holidays this week

Country: Holiday country unavailable

Data unavailable

Holiday country could not be inferred from this time zone yet.

Copy this result

Copy the calculated week result or a shareable URL with this date, time zone, and week system.

Continue planning this ISO week

2027-W01 runs from 2027-01-04 to 2027-01-10. Open all seven dates, download a calendar event, or use the ISO week-year in your spreadsheet.

Open 2027-W01 dates and calendar export View the 2027 ISO week-year calendar Use ISO week-year formulas in Excel

ISO date-to-week examples around New Year

The ISO week-year can differ from the calendar year. January 1–3, 2027 belong to 2026-W53; 2027-W01 starts on Monday, January 4. These fixed examples use UTC and ISO weeks.

Two calendar dates on either side of the 2026–2027 ISO week-year boundary
Calendar date ISO week-year and week Monday–Sunday date range
Try 2027-01-01 2026-W53 2026-12-28 to 2027-01-03
Try 2027-01-04 2027-W01 2027-01-04 to 2027-01-10