When I was verifying the issue from Angola, AO-599, I noticed that it's valid also in the Core project. It's impossible to create new processing periods when one's computer's timezone is GMT-8 on Chrome. On Firefox, periods can be created. With GMT+1, everything is fine on both browsers. Please look at the screenshots.
Set your computer's timezone to GMT-8 (i.e. Seattle time).
Log into the application as administrator.
Go to Administration > Processing Schedules.
Click the "Edit" button next to the SCH003 processing schedule.
Try to add the Jan2026 processing schedule with 31/01/2026.
The Start Date changes automatically to 31/12/2025 and the period is not created. The errors from screenshots occur.