zulu time??

Today I was teaching my intermediate students about time, how to tell the time, days, months, years and language related to time. I told them that in English the clock has 12 hours and you add am/pm if needed and that mostly the military in US movies use the 24 hours clock such as 12hundred hours.
So they told me that they've seen a movie when the hero said in the radio something like "14hundred Zulu", and they asked me what does that means.
I told them that I know that Zulu is code for the letter z, the same international code that has Charlie for the c or tango for the t. However it doesn't make any sense to tell the time plus a zed. I also told them that I'd try to find it out anyway.

any idea?