There is a Bug with Proton Sheets (Non-US based location) where when you double-click a date cell and attempt to change it via the drop down calendar selection box, it will mix up the day and month selection and give you the wrong date (I imagine it is mixing up day/month in the order due to a mixup between US and European date formats, juggling mm/dd/yyyy vs dd/mm/yyyy respectively).

Example:

I begin with a "Long Date" date format in my cell and have:

Thursday, January 1 2026

I then double click to bring up the calendar drop down and select the 6th of January. This updates the cell to be:

Monday, June 1 2026

The logic is taking the input of 6/1/2026 (Non-US format) from my calendar selection and translating it into the US format, giving me June 1st, not January 6th.

Further when I select a date above the 12th (out of bounds month in the US format) it breaks the logic and it gives me a slash/separated non-date result.

So from my new date of Monday, June 1 2026, I select June 18th from the calendar drop down and get:

18/6/2026

This result appears on the left-hand side of the cell, no longer indicated to be a date, it appears to be plain text. Double clicking does not bring up the date calendar. As such it appears to have broken the logic and ceased to be a date.