Hi everyone,
just an update: There were many cases when one invalid entry (with obviously wrong/mismatched dates) in the spreadsheet breaks the charts and the estimates. Moreover, Google Forms does not provide enough flexibility to tell people right away that they're entering invalid dates. As a workaround, I've implemented a filter, so that obviously invalid entries won't affect the statistics, the charts and the estimates anymore. It's not a really comprehensive solution, because sometimes even a human can't tell if there's a typo in a date, - but it helps a lot.
Validation error descriptions are now visible in the "Validation Errors" column (index AU), so diligent entry owners can read it and fix their entry. There are not so many invalid entries, so most probably validation error value for your entry will be empty - which means your entry is fine.
Before today, an older built-in functionality has been used to indicate errors with a red triangle in a corner of an invalid cell. It has been slowing down the spreadsheet and it's now replaced by the new optimized implementation, so, as a bonus, the spreadsheet just got a bit faster
