Created attachment 101521 [details]
Calendar is correct ONLY because Time field is NOT using Oz or NZ.

Overview: 
In Australia ["Oz"] & New Zealand ["NZ"], weeks begin on Mondays, not Sundays, but this widget incorrectly shows Sundays as Day 1, unless "Time" field is NOT set for Oz or NZ.

Steps to Reproduce: 
1. Select "Australia - Australian English (en_AU)" in the widget's "Set Time Format..." dialog's "Region" field.
2. Leave "Detailed Settings" inactive.
           ---OR---
1. Select "Australia - Australian English (en_AU)" in the widget's "Set Time Format..." dialog's "Region" field.
2. Select "Detailed Settings".
2a. Select "Australia - Australian English (en_AU)" also in all five individual fields of this section.
           ---THEN---
3. Left click on Time widget to show calendar.

Actual Results: 
Calendar so-created then INCORRECTLY shows weeks beginning on Sundays.

Expected Results: 
Calendar widget should show weeks beginning on Mondays.

Discovered Workaround:
i found that by retaining "Australia - Australian English (en_AU)" in the widget's "Set Time Format..." dialog's "Region" field, & manually setting it also in all individual fields below, EXCEPT "Time", then choosing somewhere DIFFERENT than Oz or NZ for "Time", the widget's calendar then correctly shows weeks beginning Mondays. Eg, if i instead use UK, or Norfolk Island, or Anguilla [etc (i did not try any others)], the widget's calendar then becomes correct. See my picture; the associated calendar is correct ONLY because "Time" field is NOT using Oz or NZ.
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Maui Linux Aurora Plasma 5.8.0.