The ability to tell time is an important skill. Being able to write the time in an easily understandable format is equally important if you wish to communicate effectively. The U.S. states time in 12-hour increments, unlike some other places which use a 24-hour clock. Factors like daylight-saving time and time zones can make the act of communicating meeting times slightly more complicated, but you can minimize the confusion by using the correct form to express the meeting time.
Write the time with the hour, then a colon, and then the minutes. For example, you might ask to meet with someone at twelve fifteen by writing 12:15.
Express whether you wish to meet in the morning or evening by using the terms "a.m." or "p.m." Thus a meeting at lunch time would be at 12:15 p.m.; a.m stands for ante meridiem or "before noon," while p.m. stands for "post meridiem" or "after noon."
Writing a description of the clock's hands hitting 12 is slightly different. Since a.m. and p.m. describe before and after noon, there is no description for 12 o'clock. In this case, simply express it as noon or midnight. No numbers are required.