Before noon.
Example:
On great ancient sundials the shadow crossed the central line at noon, dramatically marking the shift from ante meridiem to post meridiem.
Ante meridiem is almost always abbreviated as a.m. The term is spelled out only in the most formal contexts, such as laws and statutes. There is controversy about the use of a.m. and its counterpart p.m., for post meridiem, when referring to twelve o'clock. Some people have argued that 12:00 a.m. means midnight and 12:00 p.m. means noon; others have insisted the opposite. There has never been any general agreement. If you want to avoid confusion, use noon or midnight, either alone or preceded by 12:00.