My daughter would admonish me that it should read 1515. Is it the same in all branches? Small point but something to think about.
You would never use a colon in military (or police) time. But the guy seems to be something of a "free-form" writer who's not too picky about spelling so maybe it's a typo.
The only difference i know if is that the army puts the word "hours" into it. Us navy medicine folk get to be a bit lazy, but most grunts are usually gungho about those things, strange "mistake"