Week Numbers for 2021 — ISO Week List

Week numbers for 2021

2021 has 52 ISO weeks. Week 1 starts on $Jan 04, 2021 and week $52 ends on $Jan 02, 2022. Each week below shows its ISO week number, the Monday start date, the Sunday end date, and the Unix epoch timestamp for the start of the week (00:00 UTC on Monday). Weeks are numbered per ISO 8601, where week 1 is the week containing the first Thursday of the year and every week runs Monday to Sunday.

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The "End epoch (UTC)" column shows the Unix timestamp for 00:00 UTC on the Sunday of that week, which is the first second of the last day of the week. The last second of the week is one second before the Monday start of the next week.

How to read this table

• Week # — the ISO 8601 week number for that week.
• ISO year — the ISO week-numbering year this week belongs to (can differ from the calendar year for a few edge days).
• Monday start — the date of the Monday that starts the week, in UTC.
• Sunday end — the date of the Sunday that ends the week, in UTC.
• Start epoch (UTC) — Unix timestamp for 00:00 UTC on Monday.
• End epoch (UTC) — Unix timestamp for 00:00 UTC on Sunday.

Why week numbers pair with epoch time

A Unix timestamp is a single number for an instant. The ISO week number is a label for a seven-day span. Converting between them is common in reporting: a log timestamp becomes "week 36", and a week label becomes the epoch range for that week. Use the timestamps in this table with the epoch converter to move between a week and its Unix time in either direction.

• Epoch $1609718400 → Monday start of ISO week 1 of $2021.
• Epoch $1610236800 → Sunday end of ISO week 1 of $2021.
• Epoch $1640563200 → Monday start of ISO week $52 of $2021.
• Epoch $1641081600 → Sunday end of ISO week $52 of $2021.

52-week and 53-week years

Under ISO 8601, a year has 53 weeks when the year starts on a Thursday, or when it is a leap year that starts on a Wednesday. Otherwise it has 52 weeks. That is why the number of weeks in a year is not always 52 — it is 52 most years, and 53 about once every five or six years.

• $2021 is a $52-week year under ISO 8601.
• Week 1 of $2021 starts on $Jan 04, 2021.
• Week $52 of $2021 ends on $Jan 02, 2022.

Comparing week numbers across years

Week numbers shift from year to year because each year starts on a different day of the week. A date like March 1 can be in week 9 in one year and week 10 in another. Comparing the week-number tables for consecutive years is the fastest way to see that shift:

• Week numbers for 2024 (52 weeks)
• Week numbers for 2025 (52 weeks)
• Week numbers for 2026 (53 weeks)
• Week numbers for 2020
• Week numbers for 2022

Examples

• January 1, $2021 falls in ISO week $1 of $2021.
• Today ($Sep $18, $2021) falls in ISO week $— of $2021 — $n/a.
• ISO week 1 of $2021 runs from $Jan 04, 2021 to $Jan 10, 2021.
• ISO week $52 of $2021 runs from $Dec 27, 2021 to $Jan 02, 2022.