Hi Emeric , 

Though your question was posted a long time ago, I still want to share the solution I found for this issue. It might help you and other Ninox users.

Background:

The "problem" with the date format changing in the Ninox app after updating to macOS Ventura is caused by Apple. While they changed the macOS settings panel, they removed the option to easily edit date/time formatting. Whoops! It looks like Ninox relies heavily on the medium time format for your date field, and guess what: that one is not easy to change with the new macOS settings. It would be nice if Ninox would change this in the future, so it's not depending on this setting anymore, but for now this is the cards we're dealing with.

Solution:

I found some info on this Reddit page, so credits to the folks over there. There was more information, but it's not on that page anymore. I did make a copy of it and I'll add this extra info after the Solution.

First of all: make sure you know what you're doing once you start digging into macOS with the Terminal!

Go to the macOS terminal and enter the following, then hit enter: 

defaults write -g AppleICUDateFormatStrings -dict 2 'dd-MM-yyyy'

The 'dd-MM-yyy' dictates what the format is. If you want to use another format, please find the extra info below.

Extra info:

(I copied and saved this information without a link to the original poster, so unfortunately I'm not able to credit the right person anymore.)

This is a case of "everything old is new again". In Tiger or Leopard (I forget!) Apple removed the Advanced time/date format settings, so we all had to learn how to set them manually. Then, one or two major versions later, they added it back again. Anyway…

You're looking at setting at least three keys in the NSGlobal domain:

• AppleICUNumberSymbols

• AppleICUDateFormatStrings

• AppleICUTimeFormatStrings

They're all dictionaries. (Some of) the possible keys of AppleICUNumberSymbols are:

• 0 — decimal separator for numbers

• 1 — thousands separator for numbers

• 4 — starting character for digits

• 8 — currency symbol

• 10 — decimal separator for currency

• 17 — thousands separator for currency

Example: If want to use a space as thousands separator and a . as decimal separator, you have to do:

defaults write -g AppleICUNumberSymbols -dict 0 '.' 1 ' ' 10 '.' 17 ' '

(Note the -g flag to specify the global domain!)

AppleICUDateFormatStrings has four possible keys:

• 1 — short date format

• 2 — medium date format

• 3 — long date format

• 4 — date date format

So, to set the short date in ISO8601:

defaults write -g AppleICUDateFormatStrings -dict 1 'y-MM-dd'

(I may be wrong about the exact format; please go and look it up in the ICU Documentation!)

AppleICUTimeFormatStrings works analogously to AppleICUDateFormatStrings.

You may or may not need to log out and back in — or even reboot? — after making these settings. Note that the values in the dropdown menus in System Settings will not change, but the bit of grey example text in the "Language & Region" panel showing the dates and times will change to reflect your new formats.

Sources of information:

• http://www.macfreek.nl/memory/Decimal_Seperator_in_Mac_OS_X

• http://hints.macworld.com/article.php?story=20080403055310704

• https://gist.github.com/chris-79/654bfcd252280230fa82

• https://opensource.apple.com/source/CF/CF-550/CFDateFormatter.c.auto.html