What determines the date format in text widgets?
My organisation has Australian Date formats. My profile has Australian Date formats.
This data is sourced from the classic web map where the pop-up has the date format as dd/mm/yyyy
At least the new map viewer has mmm dd yyyy, classic didn't let you choose that date format and have show time.
Fully empathise with your question, dates can be really frustrating - especially when you've set your AGOL org web map, device, and browser to the correct format. I wish, when set, the desired date format cascaded across all hosted layers, applications, and maps.
I raised a question regarding the start of a week - I wanted to have the day as Monday to reflect a working week rather than the fixed Sunday that is in filters ie; "Date is This Week"