Convert civilian time among 24-hour (military) time, Greenwich Mean Time (GMT), and Julian calendar.

Conversion should include 

• maintaining four-number consistency
• adding consecutive hours after noon in military time (e.g., 1 p.m. becomes 13:00 in military time) 
• subtracting 12 from any number of 13-hundred or higher to convert from military to regular time. 

Process/Skill Questions: 

• What is the mathematical process to convert civilian time to military time?
• When it is 7:00 am GMT, what time is it in CST in the United States?
• Why is a 24-hour (military) time used in aviation?