Calendar Week Number

WHAT IT DOES

Calendar Week Number is a browser extension that displays the current ISO week number in a floating panel, along with the current date and day of the week. The week number is shown prominently in a large, clear format with the year, and updates automatically every minute. You can copy the week number (e.g., "Week 42 (2026)") or the full date information to your clipboard with one click. The tool is ideal for project managers, software developers, product teams, logistics professionals, and anyone who works with week-based planning, sprint planning, or ISO week numbering systems.

WHERE IT RUNS

Calendar Week Number runs on any webpage, overlaying a floating panel that does not interfere with page content. It operates entirely in your browser as a lightweight userscript, calculating the ISO week number using JavaScript's Date object. It works on any page, whether static or dynamic, with no external dependencies. All data is calculated locally—no data is ever sent to any server.

HOW TO USE

After installation, open the floating panel by clicking the extension icon. The panel displays the current week number prominently (e.g., "W42") along with the formatted date (e.g., "Oct 15, 2026") and the full day of the week (e.g., "Thursday · Thu"). The week number updates automatically every minute to ensure accuracy. Click the "Copy Week" button to copy the week number and year to your clipboard in the format "Week 42 (2026)". Click the "Copy Full" button to copy both the week information and the full date. Click the "Refresh" button to manually update the display. The panel can be dragged to any position on the screen.

REAL MECHANISM — WHY THIS WORKS

Most week number tools rely on external APIs or simple date calculations that may not follow the ISO 8601 standard correctly, leading to off-by-one errors at the beginning and end of the year. Calendar Week Number uses a strict ISO 8601 calculation algorithm implemented entirely in JavaScript. The algorithm works by first finding the Thursday of the current week (ISO weeks are defined by the week containing the Thursday). It then finds January 4 of the relevant year (which is always in week 1) and calculates the number of weeks between those dates. The calculation handles year boundaries correctly—weeks that span December and January are assigned to the year of the Thursday. The result is the official ISO week number as defined in ISO 8601. The tool also computes the ISO year, which may differ from the calendar year for weeks at the beginning or end of the year. The display updates every minute using setInterval, and also updates when the browser tab becomes visible again. This approach ensures accurate, reliable, and fully private ISO week number calculation without requiring any external services or network access.

THE PANEL

The floating panel is fully draggable and remembers its position per site, snapping to the edges of the viewport when dragged near them. You can collapse the panel to a small icon to minimize screen distraction and expand it back with a single click. The panel is resizable from the bottom-right corner, giving you more space to view the display. Press the Escape key to close the panel instantly. The panel remains open across page navigation within the same domain, maintaining your current view state. All preferences—panel position—are saved per domain for a seamless experience.

PLEASE NOTE

Calendar Week Number requires the free BotGentz extension to be installed and enabled, as it relies on the cross-extension messaging API provided by BotGentz for DOM access permissions. The week number calculation follows the ISO 8601 standard, which defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday and end on Sunday. All data is calculated locally in your browser using JavaScript's Date object. No data is ever sent to any server. The extension does not read, store, or transmit any personal information. It only calculates and displays the current week number locally at your request.