[diggan]

Is this the datetime format they use onboard on the ISS or why is it being used in this webui?

This is the first time I come across this particular format, but also never near space-stuff so wouldn't surprise me if they use some specific format for some specific purpose.

[dr_orpheus]

Yes, using Day of Year for time GUIs and measurements is pretty common in spacecraft operations. Generally easier to do/plan operations for things like "we need to perform this maintenance activity every 5 days"

[pyrolux]

That's very common in ISS operation schedules - next spacewalk is on GMT xx next launch on GMT xx, etc

I expect it's easier and more useful to track days until event or days since event