Calendar days are wrong by one day

[thegreyshadow]

Using event calendar 76 currently with this setup:

Operating System: Slackware 15.0-current
KDE Plasma Version: 5.27.9
KDE Frameworks Version: 5.111.0
Qt Version: 5.15.11
Kernel Version: 6.1.59 (64-bit)
Graphics Platform: Wayland

Memory: 15,5 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 620

Description:
The calendar part on Event Calendar shows the days wrong. Even though I have selected to display the weeks starting on Sundays, still the calendar shows weeks starting on Monday but the weekday labels are off by one day.

The attached screenshot shows today's date, Wednesday, October 25th, 2023, as being a Tuesday (since it is rendered in Spanish, the label is "mar", as in "martes" = Tuesdays.

The tooltip information is still correct, though.

I request a fix to this since it is very annoying. Thanks in advance!