If you work with weekly surveillance data, you know the drill. A line list lands in your inbox with three hundred dates in it, and someone needs the MMWR week for every single one before the Friday call. Not the calendar week. The epiweek. The one the CDC uses, where the week starts on a Sunday and week one is the first week with at least four days in January.
I used to do it the slow way. A spreadsheet formula here, a lookup table there, and a quiet prayer that I hadn't mixed up the year boundary. Because that's where it always goes wrong. A case dated 29 December might belong to week one of the next year, and a case on 2 January might still sit in week 52 of the old one. Get that wrong and your weekly counts shift, your charts lie, and nobody notices until the report is already out the door.
The annoying part is that the conversion itself is simple once you know the rule. The hard part is doing it five hundred times without slipping once. That's a job for a tool, not for concentration.
So I built a small one that does the boring bit. You paste a column of dates, and it hands back the epiweek year and week number for each of them. That's it. No sign-up, no install, nothing to download. It runs in the browser, so the data never leaves your machine, which matters when the dates belong to real cases.
It handles the year-end edge cases properly, which is the whole point. Dates near New Year land in the right epiweek year, not just the calendar year they happen to sit in. And because it's built for bulk work, you can paste a whole column at once instead of converting one date at a time and losing your afternoon to it.
Who is this actually for? Anyone who files weekly reports to a health department, anyone cleaning surveillance data in R or Python who wants a quick check against their own code, and anyone who has ever stared at a date like 2026-01-03 and wondered which week it really belongs to. If you've been burned by the year boundary before, you know exactly what I mean.
The CDC's MMWR week system exists so that every state, every lab and every hospital counts the same week the same way. When your week numbers match theirs, your numbers line up with the national picture. When they don't, you're arguing about whose chart is right instead of looking at the trend.
I've stopped converting dates by hand, and honestly I should have stopped years ago. If your Friday involves a spreadsheet full of dates and a deadline, try the converter and see how much of your afternoon you get back: https://toolbrewlab.com/bulk-mmwr-epiweek-converter/
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