I Thought Week Numbers Were Simple — Then I Missed a Client Deadline by 7 Days

It was month three of a six-month project for a manufacturing client in Stuttgart. Things were going fine. We had a shared timeline, weekly check-ins, the whole thing. The contract said deliverables were due by Week 28.

I marked my calendar. Week 28. Plenty of time.

Then I got a call on a Monday morning — July 6th. My client's voice was tight. "Where's the deliverable? It was due last week."

I stared at my screen. Last week was Week 27. I was sure of it. July 6th was the Monday of Week 28. I still had seven days.

I didn't. I had zero days. The deadline was already gone.

The problem hiding in plain sight

Here's what happened. My client, based in Germany, used ISO 8601 — the international standard where weeks start on Monday and Week 1 is the week that contains the first Thursday of the year. My calendar? It used the US system. Sunday start. Week 1 contains January 1st, period.

So when we both said "Week 28," we were talking about two different sets of seven days. Their Week 28 ended on July 5th. Mine started on July 6th.

That one-day overlap was where the whole thing fell apart.

"When we both said 'Week 28,' we were talking about two different sets of seven days."

I had spent three months thinking we were perfectly aligned. Every status update, every "see you Week 28," every calendar invite — all of it was based on a shared assumption that didn't exist.

What I should have asked on day one

Looking back, the fix was embarrassingly simple. I should have asked one question during the kickoff call: "When you say Week 28, which week numbering system are you using?"

I didn't ask because I didn't know there were different systems. I had used week numbers for years — in project plans, in reports, in casual conversation — and never once questioned whether "Week 28" meant the same thing to everyone in the room.

That was the real mistake. Not the calendar setting. The assumption.

The three systems you need to know about

After this mess, I dug into it. Turns out there are three main ways people count weeks, and they don't agree with each other:

ISO 8601. Used in most of Europe, plus much of Asia. Weeks start Monday. Week 1 is the week containing the first Thursday of the year. A year can have 52 or 53 weeks. This is the one my German client was using.

US System. Weeks start Sunday. Week 1 is the week that contains January 1st. Simple, but incompatible with ISO. This is what my default calendar used.

Simple count. January 1st is always Day 1 of Week 1, no matter what day of the week it falls on. Some payroll systems and manufacturing schedules use this because it's easy to code.

The gap between these systems can be anywhere from 0 to 7 days depending on the year and the specific week. In my case, it was exactly 7.

How we fixed it (and kept the client)

I'll be honest — that Monday morning call was rough. I didn't make excuses. I told them exactly what happened: I used the wrong week numbering system, the deadline had already passed, and I needed 48 hours to deliver.

They weren't happy. But they appreciated that I could explain why it happened without blaming anyone else. I sent them a quick summary of the ISO vs US difference, we agreed to use ISO for everything going forward, and I delivered the work by Wednesday.

We finished the project. They renewed the contract. But I never forgot that feeling of looking at my calendar and realizing I was already late for something I thought I had a full week to finish.

What I do now

Every project starts with this line in the kickoff notes: "All week references use ISO 8601 (Monday start, Week 1 = first Thursday). Confirm if your organization uses a different system."

It takes five seconds. It has saved me — and my clients — at least three more of these situations since that first one.

I also keep a browser tab open to WeekNumber.cc now. I can punch in any date and see what week it is under ISO, US, or simple count — side by side. No more assumptions.