A time designation of 1600 hours is the equivalent of four o'clock in the afternoon. Such a designation is identified as military time.
The military uses a standardized method of timekeeping in which the hours are numbered from 00 to 24. Midnight is written as either 0000 or 2400 hours. Regular or civilian time also uses a standardized method, but the hours are numbered from 1 to 12. Regular time includes the "a.m." and "p.m" indicators to distinguish between day and night hours, which are unnecessary in military timekeeping.
Military operations will often use the additional identifier of "UT" or "UTC" to indicate Universal Time, or Zulu Time, which is useful for coordinating joint military operations that occur in different regions of the world. Universal Time is based on Greenwich Mean Time (GMT), which is the time at the zero meridian.