TIMEVALUE returns the internal time number from a text enclosed by quotes and which may show a possible time entry format.
The internal number indicated as a decimal is the result of the date system used under LibreOffice to calculate date entries.
If the text string also includes a year, month, or day, TIMEVALUE only returns the fractional part of the conversion.
TIMEVALUE("4PM") returns 0.67. When formatting in time format HH:MM:SS, you then get 16:00:00.
TIMEVALUE("24:00") returns 1. If you use the HH:MM:SS time format, the value is 00:00:00.