With this number format code you can represent any number as a time difference. The neat equations and formulas editor Math is LibreOffice's formula editor, and can be invoked in your text documents, spreadsheets, presentations and drawings, enabling you to insert perfectly formatted mathematical and scientific formulas. An easy example would be to use :MM:SS (am/pm makes no sense for time differences) which tells the number formatter that you don't want to calcuate times modulo 24 hours. In your case you want to show negative time (or time differences) so you want to represent negative numbers like -0.1 as -02:24:00 which requires you to adapt the number format code. The number format code that corresponds to 12:00:00 AM is HH:MM::SS AM/PM which says that the time should be calculated modulo 24 hours and AM/PM applied automatically. The second important thing to understand is that the number format code tells you how your value is interpreted. Now if you add the default time format you'll get something like 12:00:00 AM (for an en-US locale, for other locates the representation looks different). The value 1 represents 24 hours when formatted as time or one day past the zero date (can be changed in the options). The important part for working with time values as well as dates in LibreOffice (same is true for Excel) is to understand that these are just normal numbers with a special number format.

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