How to enter time in 4 digit (not 6 digit) format?

I enter a time such as “19:10”, which is correct, Calc then converts the time to “19:10:00”. The time is now erroneous. The time should be 19:10. There should be NO seconds after the minutes, the seconds MUST remain unspecified, otherwise the data can not be validated as correct.

A recognised time value is a number. How does your validation distinguish between twice the same number?

In LibreOffice, there’s no way to store data in a date/time format with some parts unspecified. They will always have some value, even if 0. You can avoid showing it, but it will still be there.

You may also choose to store data in textual format (to do that, you need to format cells as text before entering the data, or prepend your data with ').

But you didn’t tell what “validation” you are talking about; so possibly you just need to change the validation procedure instead. E.g., you might want to compare not the times in cells, but formulas like =TIME(HOUR(B2);MINUTE(B2);0)=TIME(HOUR(C2);MINUTE(C2);0).

Stored is the numeric value for 19:10, but the default display format for time is HH:MM:SS, if you don’t want seconds to be displayed then apply the number format HH:MM to the cell, use Ctrl+1 or menu Format → Cells…