Download of ISO8601 1.0.2 for Mac was available from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. We recommend checking the downloaded files with any free antivirus. This free software for Mac OS X is a product of Christoph Krey. The software lies within Productivity Tools, more precisely Timekeeping.
From the developer:
ISO8601 is an UTC ISO8601 clock for your status bar on macOS. This simple application is created to display the UTC ISO weekday, date, and time in the status bar. Obviously, you can customize the appearance of time references in the status bar, by selecting or deselecting the time information you are not interested in.
You may want to check out more software for Mac, such as Telescope or UTC Time, which might be related to ISO8601.