commit 1774ff75d7f1db9401c0082ebddc4c7f5b4e89cb
parent 958be8507462f6368810899ccbcfdcc33f72368f
Author: Eamon Caddigan <eamon.caddigan@gmail.com>
Date: Tue, 1 Aug 2023 20:31:50 -0700
Simple post about week numbers
Diffstat:
1 file changed, 71 insertions(+), 0 deletions(-)
diff --git a/content/posts/iso-week/index.md b/content/posts/iso-week/index.md
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+---
+title: "ISO Week"
+date: 2023-08-02T19:48:25-07:00
+draft: false
+categories:
+- Programming
+---
+
+Every nerd knows that [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) is
+the "correct" way to display dates, but those who dig into the standard know
+that it encompasses more than YYYY-MM-DD format[^isotime]. One of the
+stranger conformant formats uses week number; today's date in this format is
+2023-W31-3; i.e., today is the third day of the 31st week of 2023.
+
+I've been using this format for note-taking for a couple years. I find it
+easier to track to-dos with this because it's so easy to tell at a glance
+how many weeks and days I have to finish something.
+
+In a POSIX command-line, you can print ISO-8601 week date with:
+
+```sh
+date +%G-W%V-%u
+```
+In Vim, use:
+
+```vim
+:put =strftime('%G-W%V-%u')
+```
+
+I'm not here to convince you to _give up months_ or anything; there are
+plenty of confusing things about this format:
+
+* Nobody will know what you mean when you talk about "week 31"
+* Years have different numbers of weeks (52 or 53)
+* Unless the year happens to begin on Monday, either the beginning of
+ January or end of December fall on a different year than the rest of that
+ month (which is why you need `%G` instead of `%Y` in the date format
+ string)
+
+## ISO and epi weeks
+
+I worked with week numbers when I helped conduct [research on Influenza-like
+illness](https://doi.org/10.1001/jamanetworkopen.2022.11958). The CDC
+publishes a [weekly surveillance
+report](https://www.cdc.gov/flu/weekly/index.htm), and week 40 is
+traditionally recognized as the beginning of each flu season. The CDC also
+use week numbers to track other diseases, such as Covid-19.
+
+Unfortunately, CDC weeks (aka "epidemiological weeks", or "epi weeks")
+aren't ISO weeks; the former start on Sunday while the latter start on
+Monday, which means that Sundays will have different week numbers between
+the two systems. Except, more confusingly, when January 1 falls on a
+Thursday (as it will in 2026)---then _only_ Sunday has the same week number,
+and the epi week will be one behind the ISO week for the other days. This
+table shows the ISO week and epi week of the first Monday of the year,
+ordered by the day that the year starts, and illustrates some of the issues
+with week-based dates.
+
+January 1 | First Monday | ISO week | Epi week
+----------|--------------|----------|---------
+Monday | January 1 | 1 | 1
+Tuesday | January 7 | 2 | 2
+Wednesday | January 6 | 2 | 2
+Thursday | January 5 | 2 | 1
+Friday | January 4 | 1 | 1
+Saturday | January 3 | 1 | 1
+Sunday | January 2 | 1 | 1
+
+[^isotime]: [Here's a cool
+visualization](https://ijmacd.github.io/rfc3339-iso8601/) of date and time
+formats.
