Dear María Elena:
Below you will find an academic answer taken from: www.thefreedictionary.com I hope it helps.
The
12-hour clock is a time conversion convention in which the 24
hours of the
day are divided into two periods
[1] called
ante meridiem (
a.m.,
English: "before midday") and
post meridiem (
p.m., English: "after midday").
[2] Each period consists of 12 hours numbered:
12 (acting as zero),
[3] 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and
11.
To avoid confusion, the correct designation for twelve o'clock is 12 noon or 12 midnight. Alternatively, the twenty-four-hour-clock system may be used. The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line). At 12 noon the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither "ante-" nor "post-".
[9]
