Transcript proofreaders will normally suggest using four figures and removing the colon from what is often mistakenly called “military time.” And the common references support that suggestion.
BGGP says to use four figures with no punctuation “[f]or a 24-hour time system as used in the military.”
LMEG says that “[w]hen speakers use military time,” do not use punctuation unless seconds are designated, in which case use colons.
GRM suggests that “[i]n the ’24-hour’ system of expressing clock time,” all times are expressed in four digits with no colon.
CMOS says that in the “twenty-four-hour system of expressing time,” four digits always appear with no punctuation between hours and minutes.
I can see recommending the use four digits. The U.S. is one of the few nations that still primarily uses the 12-hour, A.M./P.M. system, so a reference to 9:32, without a designator, could be in the morning or in the evening, whereas 09:32 makes it clearly a reference to the morning time.
Prohibiting a colon as a separator, though, makes no sense. I see no way in which the use of a colon lessens accuracy or clarity. In fact, I’d argue the use of a colon increases readability by giving the reader an immediate cue that the figure most likely represents a time of day.
All that just to say this. I’ll initially note what the rules say about so-called “military time,” but view it — especially the use of a colon — as a reporter preference. I will be more concerned with clarity and consistency.
As a “side peeve,” 24-hour time and “military time” (in the U.S. & allied nations) are not the same. Military time is an adaptation of 24-hour time, and yes, the military standard requires the use of four figures with no punctuation. But if we are going to follow the standard, let’s follow it: It also requires an appended timezone letter code.
“Times in messages are expressed as four figures followed by a time zone letter. The first pair of figures indicates the hour on a 24 hour clock. The second pair indicates the minutes past the hour.
ACP 121(I), COMMUNICATION INSTRUCTIONS – GENERAL, Chapter 3, No. 326
EXAMPLE: 1535Z”
You seem to have argued against yourself in the example of 9:32 (with colon) in the U.S., where that could be confusing if it’s not made clear that it’s morning. I would stick with a consistent rule of not using the colon between hour and minute (per LMEG). 24-hour time is used, for instance, in medical records. In a deposition, etc., if time is being expressed that way, I wouldn’t use a colon — except between minutes and seconds, as LMEG notes. Also, I prefer to refer to “24-hour time” instead of “military time” if it’s being used in non-military contexts.
P.S. Just for background: I’ve been proofreading for court reporters for a long time. You might add Margie Wakeman Wells (MWW) to your list of references, at least for legal document standards. She’s an alternative to LMEG followed by many court reporters. (I prefer LMEG, but MWW does have a lot of good, up-to-date information to offer.)