Created attachment 130291 [details]
sample test ods

In the attached "Calc timevalue odssample" you can see and verify my tests:
=ORARIO.VALORE("00.00") cannot anyway display as result "12.00.00m." (cell format "H.MM.SS AM/PM" or "HH.MM.SS AM/PM") where "m." stays for antimeridian.
=ORARIO.VALORE("24.00”) cannot anyway display as result "12.00.00m." (cell format "H.MM.SS AM/PM" or "HH.MM.SS AM/PM") where "m." stays for antimeridian.
Further I note that anti and post meridian espress in Calc with the letters "m." and "p." are unusual symbols (in IT we generally use “a.m. or am" and "p.m. or pm" lower or uppercase):
I translate what you can read about at the link http://tp.linux.it/data_ora_valuta.html
TIMEVALUE.
The conventional notation to indicate a time in Italian language is "o:mm:ss"
with the hour expressed in 24-hour format without leading zero.
Expressed in the syntax of the GNU program date(1) becomes: %k:%M:%S
Examples:
13:00:56
4:12
The presence of leading zeros (obtained with dates(1) by replacing the %k with %H) is allowed in case you have the need for alignment issues or ordering.
It should be noted, that only for the alignment motivation, the agreement for date(1) provides for the possibility to use the "_" and "-" modifiers to hide some digit.
Consult the man page for more information.
In case you must use the 12-hour format, morning and afternoon should be rendered as "a.m." and "p.m.". Any other expression (AM/PM or A.M./P.M and so on) is to be avoided.
Also in case you must use the 12-hour format, the issue date(1) to use for the hour is %-I