It is possible to format times to use 12-hour AM/PM format, instead of 24-hours. However, the help [1] does not tell that in the respective "Time Formats" section. An example of the format string is: H:MM AM/PM and the "AM/PM" (five characters, AM then slash then PM) is the exact code to get the 12-hour format; e.g., "AM / PM" or "AMPM" wouldn't work. A Basic code like MsgBox Format(Time(), "HH:MM AM/PM") gives me at this moment 09:09 PM [1] https://help.libreoffice.org/latest/en-US/text/shared/01/05020301.html
Pierre F committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/2694856dafe3e7850b966583e093e62b05dc099d add paragraph about "AM/PM" tdf#157603 05020301.xhp