Converting a 12-hour time to 24-hour time is straight forward, though confusing. From 1:00 PM to 11:59 PM you add 12 hours, and from 12:00 AM (midnight) to 12:59 AM you subtract 12 hours.

Or you can simply use the following calculator to convert to or from either system. Simply pick the hour (the minutes do not change) in either system and the other will change to match.

Notes:
The 24-hour clock is a time keeping convention where the day runs from midnight to midnight and is split into 24 hours, from hour 0 to hour 23. It is the most common system in use in the world, and is the international standard notation of time. (ISO 8601)

In the U.S. and Canada the 12-hour AM/PM format is still more commonly used. In these countries the 24-hour format is called 'Military time'.