The day begins at midnight, 00:00. It is a military time (24-hour clock). The hours before 10 am start with a 0; so 1:00 am is 0100 and said as zero-one-hundred and so forth. How it is pronounced greatly depends on which branch of the military you’ve serve. In the Navy and Marine Corps, there was a lot of emphasis on pronouncing 0500 as "zero five hundred" instead of "oh five hundred," etc., and, as a result, we said "zero dark thirty" instead of "oh dark thirty."
And by that "0-dark-thirty in the morning" and why it is ''dark'', it means midnight. (12:30 am)